Semiconductor device and method for manufacturing the same

ABSTRACT

In an active matrix display device, electric characteristics of thin film transistors included in a circuit are important, and performance of the display device depends on the electric characteristics. Thus, by using an oxide semiconductor film including In, Ga, and Zn for an inverted staggered thin film transistor, variation in electric characteristics of the thin film transistor can be reduced. Three layers of a gate insulating film, an oxide semiconductor layer and a channel protective layer are successively formed by a sputtering method without being exposed to air. Further, in the oxide semiconductor layer, the thickness of a region overlapping with the channel protective film is larger than that of a region in contact with a conductive film.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device which has acircuit including a thin film transistor (hereinafter, referred to as aTFT) in which an oxide semiconductor film is used for a channelformation region and a manufacturing method thereof For example, thepresent invention relates to an electronic device in which anelectro-optical device typified by a liquid crystal display panel or alight-emitting display device including an organic light-emittingelement is mounted as its component.

Note that the term “semiconductor device” in this specificationindicates all the devices which can operate by using semiconductorcharacteristics, and an electro-optical device, a semiconductor circuit,and an electronic device are all included in the semiconductor devices.

DESCRIPTION OF THE RELATED ART

In recent years, active matrix display devices (such as liquid crystaldisplay devices, light-emitting display devices, or electrophoreticdisplay devices), in which a switching element of a TFT is provided foreach of display pixels arranged in a matrix, have been activelydeveloped. In the active matrix display devices, a switching element isprovided for each of pixels (or each dot), and thus, there is such anadvantage that the active matrix display device can be driven at lowervoltage than a passive matrix display device in the case where the pixeldensity is increased.

In addition, a technique has attracted attention, where a thin filmtransistor (TFT) in which an oxide semiconductor film is used for achannel formation region or the like is manufactured and such a TFT orthe like is applied to electronic devices or optical devices. Forexample, a TFT in which ZnO is used as an oxide semiconductor film or aTFT in which InGaO₃(ZnO)_(m) is used as an oxide semiconductor film canbe given. A technique in which a TFT including such an oxidesemiconductor film is formed over a light-transmitting substrate andused for a switching element or the like of an image display device isdisclosed in Patent Document 1 r Patent Document 2.

-   Patent Document 1: Japanese Published Patent Application No.    2007-123861-   Patent Document 2: Japanese Published Patent Application No.    2007-96055

SUMMARY OF THE INVENTION

In an active matrix display device, electric characteristics of thinfilm transistors included in a circuit are important, and performance ofthe display device depends on the electric characteristics. Inparticular, the threshold voltage (Vth) is important among the electriccharacteristics of the thin film transistor. When the threshold voltagevalue is high or the threshold voltage value is on the minus side evenif the field-effect mobility is high, it is difficult to control thecircuit. When a thin film transistor has a high threshold voltage valueand a large absolute value of its threshold voltage, the thin filmtransistor cannot perform switching function and may be a load when thetransistor is driven at low voltage. Further, in the case where thethreshold voltage value is on the minus side, current tends to flowbetween the source electrode and the drain electrode even when the gatevoltage is 0V; in other words, the thin film transistor tends to benormally on.

In a case of an n-channel thin film transistor, a preferable thin filmtransistor is one in which a channel is formed and drain current beginto flow after the positive voltage is applied as the gate voltage. Atransistor in which a channel is not formed unless the driving voltageis increased and a transistor in which a channel is formed and draincurrent flows even in the case of the negative voltage state areunsuitable for a thin film transistor used in a circuit.

It is an object to provide a structure in which a channel is formed witha positive threshold voltage of a gate voltage which is as close to 0Vas possible in a thin film transistor using an oxide semiconductor filmincluding In, Ga, and Zn.

Further, another object is to reduce variation in electriccharacteristics of the thin film transistor in which an oxidesemiconductor film including In, Ga, and Zn is used. In particular, in aliquid crystal display device where variation between elements is large,there is a concern that display unevenness due to variation in the TFTcharacteristics is caused.

Further, in a display device including a light-emitting element, in acase where there is large variation in ON current (Ion) of TFTs (TFTs ina driver circuit or TFTs for supplying current to light-emittingelements arranged in pixels) arranged so as to make constant currentflow to a pixel electrode, there is a concern that variation inluminance might be generated in a display screen.

SUMMARY OF THE INVENTION

The threshold voltage of a thin film transistor is considered to begreatly influenced by an interface of an oxide semiconductor layer, thatis, an interface between an oxide semiconductor layer and a gateinsulating film or an interface between an oxide semiconductor layer andan electrode.

Thus, by formation of the interface in a clean condition, in addition toimproving electric characteristics of a thin film transistor, amanufacturing process can be prevented from being complicated, so that athin film transistor provided with improved mass productivity and highperformance can be realized.

Such an interface is formed in a clean condition, and thus three layers,a gate insulating film, an oxide semiconductor layer and a channelprotective layer are successively formed by a sputtering method or aPCVD method without being exposed to air. Preferably, the three layersare successively formed under reduced pressure so that an oxidesemiconductor layer having an excellent interface can be obtained, andleakage current of a TFT in an off state can be small and a thin filmtransistor having high current-driving ability can be obtained.

One embodiment of the present invention disclosed in this specificationis a method for manufacturing a semiconductor device, comprising thesteps of: forming a gate electrode over a substrate having an insulatingsurface; forming a stacked layer in which a first insulating film isformed over the gate electrode, an oxide semiconductor layer is formedover the first insulating film, and a second insulating film is formedover the oxide semiconductor layer by sputtering without being exposedto air; forming a protective film in a region overlapping with the gateelectrode by selectively etching the second insulating film; etching anupper layer of the oxide semiconductor layer with the protective film asa mask; forming a conductive film over the oxide semiconductor layer andthe protective film; and selectively etching the conductive film withthe protective film used as an etching stopper.

The present invention can achieve at least one of the above-describedobjects.

In the above-described manufacturing process, the first insulating filmand the oxide semiconductor layer can be formed in the same chamber. Theinterface between the first insulating film and the oxide semiconductorlayer is close to a region of a channel and thus the formation in thesame chamber is effective for improvement in electric characteristics ofa TFT, e.g., improvement in electric field mobility. In addition, theinterface between the first insulating film and the oxide semiconductorlayer becomes clean, and thus variations in a threshold voltage value oron current (Ion) can be reduced. Advantages of stacking the films in thesame chamber are that the number of chambers used can be decreased andattachment of particles, moisture or the like to a substrate can beprevented in transferring the substrate from a chamber to anotherchamber.

Moreover, in the above-described manufacturing process, the firstinsulating film, the oxide semiconductor layer and the second insulatingfilm can be formed in the same chamber. The interface between the secondinsulating film and the oxide semiconductor layer is close to a path ofleakage current in an off state and thus the formation in the samechamber is effective for improvement in electric characteristics of aTFT, e.g., reduction of leakage current in an off state. In addition,the interface between the second insulating film and the oxidesemiconductor layer becomes clean as well as the interface between thefirst insulating film and the oxide semiconductor layer, and thusvariations of elements can be reduced.

Also, one feature of the above-described manufacturing process is thatthe first insulating film is formed in an atmosphere including onlyoxygen or an atmosphere including an inert gas at less than 10% andoxygen at 90% or more by a sputtering method. The target can be aninsulating material, a metal material or a semiconductor material. Thefirst insulating film including excessive oxygen can be obtained by asputtering method. Further, an oxide semiconductor layer havingexcessive oxygen is formed over the first insulating film includingexcessive oxygen, so that the interface between the films each includingexcessive oxygen can be stable and reliability of a TFT can be improved.

Further, one feature is that an oxide semiconductor target including atleast In, Ga, and Zn is used and an oxide semiconductor layer is formedunder an atmosphere including only oxygen or an atmosphere containing aninert gas at less than 10% and oxygen at 90% or more by a sputteringmethod. For the formation of the oxide semiconductor layer, a vaporphase method such as a pulsed laser deposition method (a PLD method) oran electron beam deposition method can be used, but a sputtering methodis suitable in terms of mass productivity.

Also, one feature is that the second insulating film is formed under anatmosphere including only oxygen or an atmosphere containing an inertgas at less than 10% and oxygen at 90% or more by a sputtering method.The SiOx film including excessive oxygen, and the IGZO semiconductorlayer including excessive oxygen and the channel protective layerincluding excessive oxygen are successively formed without being exposedto air, so that the interfaces between the three layers is more stablebecause the three layers are all films including excessive oxygen. Thus,the reliability of a TFT can be improved. By successive formation of thethree layers, variations in threshold voltages can be reduced,deterioration of electric characteristics can be prevented, and shift tonormally on of a TFT can be reduced, or preferably eliminated.

In this specification, the term “successive deposition” or “successiveformation” means that deposition is conducted to a substrate to beprocessed placed under an atmosphere which is controlled to be vacuum oran inert gas atmosphere (a nitrogen atmosphere or a rare gas atmosphere)at all times without being exposed to a contaminant atmosphere such asair during a series of process from a first deposition step using asputtering method or a PCVD method to a second deposition step using aPCVD method. By the successive formation, deposition can be performedwhile preventing moisture or the like from being attached again to thesubstrate to be processed which is cleaned.

Performing the process from the first deposition step to the seconddeposition step in the same chamber is within the scope of thesuccessive formation in this specification.

In addition, the following is also within the scope of the successiveformation in this specification: in the case of performing the processfrom the first deposition step to the second deposition in differentchambers, the substrate is transferred after the first deposition stepto another chamber without being exposed to air and subjected to thesecond deposition.

Note that between the first deposition step and the second depositionstep, a substrate transfer step, an alignment step, a slow-cooling step,a step of heating or cooling the substrate to a temperature which isnecessary for the second deposition step, and/or the like may beprovided. Such a process is also within the scope of the successiveformation in this specification.

A step in which liquid is used, such as a cleaning step, wet etching, orformation of a resist may be provided between the first deposition stepand the second deposition step. This case is not included in thesuccessive formation in this specification.

In addition, in the above-described manufacturing method, when or afterpattering is performed to form the channel protective film, a part ofthe oxide semiconductor layer is intentionally etched to form a regionwith a small thickness. Accordingly, by the manufacturing methodaccording to one embodiment of the present invention, as for the oxidesemiconductor layer, the thickness of a region overlapping the channelprotective film can be larger than a region overlapping the conductivefilm.

As described above, the structure of the semiconductor device obtainedby the above-described manufacturing method is also one feature of oneembodiment of the present invention. The structure includes a gateelectrode over a substrate having an insulating surface, a firstinsulating film over the gate electrode, an oxide semiconductor layerover the first insulating film, and a second insulating film and aconductive film over the oxide semiconductor layer, and the a firstregion in which the oxide semiconductor layer and the second insulatingfilm are contact with each other overlaps with at least part of the gateelectrode, and a thickness of the oxide semiconductor layer in a secondregion in which the oxide semiconductor layer and the conductive filmare in contact with each other is smaller than a thickness of the oxidesemiconductor layer in the first region.

In the above-described structure, the oxide semiconductor layer is anoxide semiconductor film including at least In, Ga and Zn. The oxidesemiconductor film including In, Ga and Zn (also referred to as an IGZOfilm) has characteristics that its hole mobility is increased as itscarrier concentration becomes higher. In addition, the oxidesemiconductor film including at least In, Ga and Zn has an electroncarrier concentration of from 1×10¹¹ cm⁻³ to 1×10¹⁷ cm⁻³ at roomtemperature in a state that a gate voltage and a source-drain voltageare not applied. If the oxide semiconductor layer of the thin filmtransistor exceeds the electron carrier concentration range, thethreshold voltage is easily shifted to a minus value, and the thin filmtransistor tends to be normally on, the state that current flows betweena source electrode and a drain electrode even at a gate voltage of 0 V.

With the above-described structure, a structure can be realized in whicha channel can be formed with a positive threshold voltage of a gatevoltage which is as close to 0V as possible in a thin film transistorusing an oxide semiconductor film including In, Ga, and Zn.

In addition, the first insulating film is used as the gate insulatingfilm, and thus a material which has excellent interface characteristicswith the oxide semiconductor layer formed thereover is preferably usedfor the first insulating film. For example, a silicon oxide film, analuminum oxide film, a magnesium oxide film, an aluminum nitride film,an yttrium oxide film, or the like formed by a sputtering method can begiven. Alternatively, a multilayer structure in which plural kinds ofsuch films are stacked may be employed. The thickness of the firstinsulating film may be determined in consideration of the use as thegate insulating film. The typical thickness of the gate insulating filmis from 50 nm to 500 nm.

The second insulating film is used as the channel protective film, andthus a material which has excellent interface characteristics with theoxide semiconductor layer formed thereunder is preferably used for thesecond insulating film. For example, a silicon oxide film, an aluminumoxide film, a magnesium oxide film, an aluminum nitride film, an yttriumoxide film, or the like formed by a sputtering method can be given.Because the second insulating film serves as the channel protectivefilm, a damage (such as thickness reduction or oxidation due to plasmaetching or etchant in etching) to a back channel, that is, a regionopposite to a face of the IGZO semiconductor layer that is in contactwith the gate insulating film can be prevented during a process, andthus the reliability of the semiconductor device can be improved.

In addition, a small amount of a halogen element such as fluorine orchlorine may be added to the first insulating film or the secondinsulating film to immobilize mobile ions of sodium or the like. Thepeak of the concentration of a halogen element to be contained in thefirst insulating film or the second insulating film is measured bysecondary ion mass spectrometory (SIMS) and is preferably in the rangeof from 1×10¹⁵ cm⁻³ to 1×10²⁰ cm⁻³. The halogen element included in thefirst insulating film or the second insulating film has an effect ofpreventing electric characteristics of a TFT from varying due to entryof mobile ions of sodium or the like from a glass substrate including analkali metal such as sodium.

The conductive film serves as a source electrode or a drain electrode.The conductive film is formed using a single layer or a stacked layer ofaluminum, copper, or an aluminum alloy to which an element to improveheat resistance or an element to prevent a hillock such as silicon,titanium, neodymium, scandium, or molybdenum is added. Among all,titanium can be given as an example of a material having excellentinterface characteristics with the oxide semiconductor layer. Inparticular, a stack of a titanium film, an aluminum film, and a titaniumfilm is used as the conductive layer so that its resistance is low, andthe titanium films sandwich the aluminum film, thereby almost preventinghillocks generated from the aluminum film. Thus, the stack is suitablefor a source electrode or a drain electrode.

Alternatively, between the gate electrode and the first insulating filmformed by a sputtering method, a silicon nitride film or a siliconnitride oxide film may be additionally provided. In other words, thegate insulating film may have a stacked structure of two or more layers,and a silicon oxide film formed by a sputtering method is preferable forthe first insulating film as a top layer in contact with the oxidesemiconductor layer, while the insulating film formed thereunder ispreferably a silicon nitride film or a silicon nitride oxide film formedby a plasma CVD method or the like. By provision of such a siliconnitride film or a silicon nitride oxide film formed by a plasma CVDmethod or the like, the film serves as an etching stopper to prevent asurface of a substrate from being etched in a manufacturing process of aTFT. The silicon nitride film or the silicon nitride oxide film canprevent mobile ions of sodium or the like included in a glass substratefrom entering a semiconductor region so that variation in electriccharacteristics of TFTs can be suppressed.

Note that a “silicon oxynitride film” in this specification means a filmthat contains more oxygen than nitrogen and includes oxygen, nitrogen,silicon, and hydrogen at concentrations ranging from 50 at. % to 70 at.%, 0.5 at. % to 15 at. %, 25 at. % to 35 at. %, and 0.1 at. % to 10 at.%, respectively. Further, a “silicon nitride oxide film” in thisspecification means a film that contains more nitrogen than oxygen andincludes oxygen, nitrogen, silicon, and hydrogen at concentrationsranging from 5 at. % to 30 at. %, 20 at. % to 55 at. %, 25 at. % to 35at. %, and 10 at. % to 30 at. %, respectively. Note that percentages ofnitrogen, oxygen, silicon, and hydrogen fall within the ranges givenabove, where the total number of atoms contained in the siliconoxynitride film or the silicon nitride oxide film is defined as 100 at.%.

Note that in this specification, a word which expresses up, down, side,perpendicular, horizontal, or the like indicates a direction based on asurface of the substrate in the case where a device is disposed on thesubstrate surface.

Because the three layers are formed successively, a structure can berealized in which a channel can be formed with a positive thresholdvoltage of a gate voltage which is as close to 0V as possible in a thinfilm transistor using an oxide semiconductor film including In, Ga, andZn. Moreover, because the three layers are formed successively,variations in threshold voltages can be reduced, deterioration ofelectric characteristics can be prevented, and shift to normally on of aTFT can be reduced, or preferably eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1E are cross-sectional views illustrating a manufacturingprocess according to one embodiment of the present invention;

FIG. 2 is a top view illustrating a multichamber type manufacturingapparatus;

FIG. 3 illustrates an example of a cross-sectional structure of a thinfilm transistor;

FIGS. 4A and 4B each illustrate an example of a cross-sectionalstructure of a thin film transistor;

FIGS. 5A and 5B are block diagrams of display devices;

FIG. 6 illustrates a structure of a signal line driver circuit;

FIG. 7 is a timing chart illustrating operation of a signal line drivercircuit;

FIG. 8 is a timing chart illustrating operation of a signal line drivercircuit;

FIG. 9 illustrates a structure of a shift register;

FIG. 10 illustrates connection in the flip flop illustrated in FIG. 9;

FIGS. 11A and 11B are a top view and a cross-sectional view of a pixelof a liquid crystal display device respectively;

FIG. 12 is a cross-sectional view of electronic paper;

FIGS. 13A and 13B are a top view and a cross-sectional view of a pixelof a light-emitting display device respectively;

FIG. 14 is an equivalent circuit diagram of a pixel;

FIGS. 15A to 15C are cross-sectional views of light-emitting elements;

FIGS. 16A and 16B are a top view and a cross-sectional view of alight-emitting module respectively;

FIGS. 17A1 and 17A2 are top views of a liquid crystal module, and FIG.17B is a cross-sectional view of the liquid crystal module;

FIG. 18 is a cross-sectional view of a liquid crystal display device;

FIGS. 19A to 19D illustrate examples of electronic devices;

FIG. 20 illustrates an example of an electronic device;

FIGS. 21A and 21B each illustrate an example of an electronic device;

FIG. 22 illustrates an example of an electronic device; and

FIG. 23 illustrates an example of an electronic device.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the present invention will now be described.

Embodiment 1

In Embodiment 1, a thin film transistor and a manufacturing processthereof are described with reference to FIGS. 1A to 1E and FIG. 2.

A gate electrode 101 is formed over a substrate 100 first, and a firstinsulating film 102 is formed so as to cover the gate electrode 101(FIG. 1A).

As the substrate 100, any of the following substrates can be used:non-alkaline glass substrates made of barium borosilicate glass,aluminoborosilicate glass, aluminosilicate glass, and the like by afusion method or a float method; ceramic substrates; plastic substrateshaving heat resistance enough to withstand a process temperature of thismanufacturing process; and the like. Alternatively, a metal substratesuch as a stainless steel alloy substrate, provided with an insulatingfilm over its surface, may also be used. The substrate 100 may have asize of 320 mm×400 mm, 370 mm×470 mm, 550 mm×650 mm, 600 mm×720 mm, 680mm×880 mm, 730 mm×920 mm, 1000 mm×1200 mm, 1100 mm×1250 mm, 1150 mm×1300mm, 1500 mm×1800 mm, 1900 mm×2200 mm, 2160 mm×2460 mm, 2400 mm×2800 mm,2850 mm×3050 mm, or the like.

Alternatively, a base insulating film may be formed over the substrate100 before the gate electrode 101 is formed. The base insulating filmcan be formed with a single layer or stacked layers of a silicon oxidefilm, a silicon nitride film, a silicon oxynitride film, and/or asilicon nitride oxide film by a CVD method, a sputtering method, or thelike.

The gate electrode 101 is formed using a metal material such astitanium, molybdenum, chromium, tantalum, tungsten, silver, gold,copper, or aluminum or an alloy material thereof. For example, the gateelectrode 101 can be formed using CuO, Cu—Mg—O, Cu—Ca—O, Cu—Mn—O, anAl—Ni alloy, as Al—Ni—La alloy, an Al—Nd alloy or the like. The gateelectrode 101 can be formed in such a manner that a conductive film isformed over the substrate 100 by a sputtering method or a vacuumevaporation method; a mask is formed over the conductive film by aphotolithography process or an inkjet method; and the conductive film isetched using the mask. Alternatively, the gate electrode 101 can beformed by discharging a conductive nanopaste of silver, gold, copper, orthe like by an inkjet method and baking the conductive nanopaste. Notethat, as barrier metal which increases adhesion of the gate electrode101 and prevents diffusion thereof to the base film, a nitride film ofthe above-mentioned metal material may be provided between the substrate100 and the gate electrode 101. The gate electrode 101 may have asingle-layer structure or a stacked structure. For example, a structurein which a molybdenum film and an aluminum film are stacked in thisorder, a structure in which a molybdenum film and an alloy film ofaluminum and neodymium are stacked in this order, a structure in which atitanium film and an aluminum film are stacked in this order, astructure in which a titanium film, an aluminum film, and a titaniumfilm are stacked in this order, or the like can be formed over thesubstrate 100.

In this case, a stacked layer of an aluminum film and a molybdenum filmis formed by a sputtering method and is selectively etched by aphotolithography process. At this time, a first photomask is used. Notethat, because a semiconductor film and/or a wiring is/are to be formedover the gate electrode 101, the gate electrode 101 is preferablyprocessed to have tapered end portions in order to prevent breakage ofthe film to be formed thereover.

The first insulating film 102 can be formed by a CVD method, asputtering method, or the like using a silicon oxide film, a siliconnitride film, a silicon oxynitride film, or a silicon nitride oxidefilm. The first insulating film 102 is the first layer of a gateinsulating film. In this case, the first insulating film 102 is asilicon nitride film formed by a plasma CVD method. The silicon nitridefilm has an effect of preventing hillock generation, when a materialthat may cause hillocks is used for the gate electrode 101.

Next, the second insulating film 103 serving as the second layer of thegate insulating film, the semiconductor film 111, and the thirdinsulating film 104 are successively formed without being exposed to air(FIG. 1B). Because the films are formed successively without beingexposed to air, high productivity is obtained and the reliability ofinterfaces between thin films is stable. In addition, the interfacesbetween the films can be formed without being contaminated byatmospheric components or contaminant impurity elements in air.

In order that the three layers are successively formed without beingexposed to air, a multichamber type manufacturing apparatus illustratedin FIG. 2 is preferably used.

At the center of the manufacturing apparatus illustrated in FIG. 2, atransfer chamber 80 equipped with a transfer mechanism (typically, atransfer robot 81) for transferring a substrate is provided. A cassettechamber 82 in which a cassette case storing a plurality of substratescarried into and out of the transfer chamber 80 is set is connected tothe transfer chamber 80 through a gate valve 83.

In addition, a plurality of treatment chambers are connected to thetransfer chamber 80 through gate valves 84 to 88. In this embodiment, anexample in which five treatment chambers are connected to the transferchamber 80 having a hexagonal top shape is illustrated. Note that, bychanging the top shape of the transfer chamber 80, the number oftreatment chambers which can be connected to the transfer chamber can bechanged. For example, three treatment chambers can be connected to atransfer chamber having a tetragonal shape, or seven treatment chamberscan be connected to a transfer chamber having an octagonal shape.

At least one treatment chamber among the five treatment chambers is asputtering chamber in which sputtering is performed. The sputteringchamber is provided with, at least inside the chamber, a sputteringtarget, a mechanism for applying electric power or a gas introductionmeans for sputtering the target, a substrate holder for holding asubstrate at a predetermined position, and the like. Further, thesputtering chamber is provided with a pressure control means with whichthe pressure in the chamber is controlled, so that the pressure isreduced in the sputtering chamber.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used for a sputtering power source, aDC sputtering method, and a pulsed DC sputtering method in which a biasis applied in a pulsed manner. An RF sputtering method is mainly used inthe case of forming an insulating film, and a DC sputtering method ismainly used in the case of forming a metal film.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can bedeposited to be stacked in the same chamber, or a plurality of kinds ofmaterials can be deposited by electric discharge at the same time in thesame chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside the chamber and used for a magnetron sputtering method, ora sputtering apparatus used for an ECR sputtering method in which plasmagenerated with the use of microwaves is used without using glowdischarge.

In the sputtering chamber, any of various sputtering methods describedabove is used as appropriate.

In addition, as a formation method, there are also a reactive sputteringmethod in which a target substance and a sputtering gas component arechemically reacted with each other during deposition to form a thincompound film thereof, and a bias sputtering method in which voltage isalso applied to a substrate during deposition.

In addition, among the five treatment chambers, one of the othertreatment chambers than the sputtering chamber is a heating chamber forpreheating or the like before sputtering, a cooling chamber for coolingafter sputtering, or a chamber for plasma treatment.

Next, an example of an operation of the manufacturing apparatus isdescribed.

A substrate cassette storing a substrate 94 whose deposition targetsurface faces downward is set in the cassette chamber 82, and thecassette chamber 82 is placed in a reduced pressure state by a vacuumexhaust means provided in the cassette chamber 82. In each of thetreatment chambers and the transfer chamber 80, the pressure is reducedin advance by a vacuum exhaust means provided in each chamber.Accordingly, during transferring the substrate between the treatmentchambers, the substrate is not exposed to air and can be kept clean.

Note that at least a gate electrode is provided in advance for thesubstrate 94 which is placed so that its deposition target surface facesdownward. For example, a base insulating film such as a silicon nitridefilm or a silicon nitride oxide film may also be provided by a plasmaCVD method between the substrate and the gate electrode. When asubstrate formed of glass containing alkali metal is used as thesubstrate 94, the base insulating film has an effect of preventingmobile ions of sodium or the like included in the substrate fromentering a semiconductor region so that variation in electriccharacteristics of TFTs can be suppressed.

Here, a substrate over which a silicon nitride film covering the gateelectrode is formed by a plasma CVD method as the first layer of a gateinsulating film is used, and the substrate corresponds to the substrate100 illustrated in FIG. 1A. In addition, the first layer of the gateinsulating film corresponds to the first insulating film 102 illustratedin FIG. 1A. The silicon nitride film formed by a plasma CVD method isdense, so that generation of pin holes or the like can be suppressed byusing this silicon nitride film as the first layer of the gateinsulating film. Although this embodiment describes an example in whichthe gate insulating film has a stacked structure, the present inventionis not limited thereto, and the gate insulating film may be a singlelayer or have a stacked structure including three or more layers.

Then, the gate valve 83 is opened and the substrate 94 which is thefirst substrate is picked up from the cassette by the transfer robot 81.After that, the gate valve 84 is opened, the substrate 94 is transferredto a first treatment chamber 89, and then, the gate valve 84 is closed.In the first treatment chamber 89, by heating the substrate 94 by aheater or a lamp, moisture or the like attached to the substrate 94 isremoved. In particular, when the gate insulating film contains moisture,there is a concern that electric characteristics of TFTs might bechanged; therefore, heating before sputtering is effective. In a casewhere moisture has been sufficiently removed at the time when thesubstrate is set in the cassette chamber 82, this heating treatment isnot necessary.

In addition, plasma treatment may be performed on the surface of thefirst layer of the gate insulating film by providing a plasma treatmentmeans in the first treatment chamber 89. Furthermore, heating forremoving moisture may be performed in the cassette chamber 82 byproviding a heating means in the cassette chamber 82.

Then, the gate valve 84 is opened and the substrate is transferred tothe transfer chamber 80 by the transfer robot 81. After that, the gatevalve 85 is opened and the substrate is transferred to a secondtreatment chamber 90, and the gate valve 85 is closed.

In this embodiment, the second treatment chamber 90 is a sputteringchamber in which sputtering is performed using an RF magnetronsputtering method. In the second treatment chamber 90, a silicon oxide(SiOx) film is deposited as the second layer of the gate insulatingfilm. As the second layer of the gate insulating film, in addition to asilicon oxide film, an aluminum oxide (Al₂O₃) film, a magnesium oxide(MgOx) film, an aluminum nitride film (AlNx) film, an yttrium oxide(YOx) film, or the like can be used. The second layer of the gateinsulating film corresponds to the second insulating film 103illustrated in FIG. 1B.

A small amount of a halogen element such as fluorine or chlorine may beadded to the second layer of the gate insulating film so as toimmobilize mobile ions of sodium or the like. As a method for adding asmall amount of a halogen element, sputtering is performed byintroducing a gas containing a halogen element into the chamber. In thecase where a gas containing a halogen element is introduced, the exhaustmeans of the chamber is needed to be provided with an abatement systemfor rendering an exhaust gas harmless. The peak of the concentration ofa halogen element to be contained in the gate insulating film ismeasured by secondary ion mass spectrometory (SIMS) and is preferably inthe range of from 1×10¹⁵ cm⁻³ to 1×10²⁰ cm⁻³ inclusive.

When the SiOx film is formed, a sputtering method in which artificialquartz is used as a target and a rare gas, typically, argon is used, ora reactive sputtering method in which single crystal silicon is used asa target and chemically reacted with an oxygen gas to obtain a SiOx filmcan be used. Here, artificial quartz is used as a target, and sputteringis performed under an atmosphere including only oxygen or an atmosphereincluding oxygen at 90% or more and Ar at 10% or less so that as muchoxygen as possible is contained in a SiOx film. Thus, a SiOx filmincluding excessive oxygen is formed.

After the SiOx film is deposited, the gate valve 85 is opened, and thesubstrate is transferred to the transfer chamber 80 by the transferrobot 81 without being exposed to air. Then, the gate valve 86 isopened, the substrate is transferred to a third treatment chamber 91,and the gate valve 86 is closed.

In this embodiment, the third treatment chamber 91 is a sputteringchamber in which sputtering is performed using a DC magnetron sputteringmethod. In the third treatment chamber 91, an oxide metal layer (IGZOsemiconductor layer) is deposited as the semiconductor film 111. In thisspecification, a semiconductor film formed using an oxide semiconductorincluding In, Ga, and Zn is also referred to as “IGZO semiconductorlayer”. As for the IGZO semiconductor layer, the composition ratio ofthe metal elements is highly flexibly set and it functions as asemiconductor layer at a wide range of mixture ratio. For example,indium oxide containing zinc oxide at 10 wt %, a material in whichindium oxide, gallium oxide, and zinc oxide are mixed in equimolarratios, and oxide containing In, Ga, and Zn at a ratio of 2.2:2.2:1.0can be given. The IGZO semiconductor layer is preferably amorphous inorder to reduce variations in electric characteristics of a thin filmtransistor. The IGZO semiconductor layer can be deposited using an oxidesemiconductor target containing In, Ga, and Zn, under a rare gasatmosphere or an oxygen atmosphere. As the deposition conditions of theIGZO semiconductor layer, an oxide semiconductor target including In,Ga, and Zn, which is eight inches in diameter (the target in whichindium oxide (In₂O₃), gallium oxide (Ga₂O₃), and zinc oxide (ZnO) aremixed in equimolar ratios and which is sintered) is used, the distancebetween a substrate and the target is 170 mm, the pressure is 0.4 Pa,the DC current is 0.5 kW, and the atmosphere is one including argon andoxygen. Here, an oxide semiconductor containing In, Ga, and Zn is usedas a target and sputtering is performed by a pulsed DC sputtering methodunder an atmosphere including only oxygen or an atmosphere includingoxygen at 90% or more and Ar at 10% or less so that as much oxygen aspossible is contained in the IGZO semiconductor layer, whereby an IGZOsemiconductor layer including excessive oxygen is formed. Sputtering isconducted to form the IGZO semiconductor layer including excessiveoxygen. DC sputtering is preferable for formation of the IGZOsemiconductor layer since dust can be reduced and the film thickness canbe even.

As described above, the SiOx film including excessive oxygen and theIGZO semiconductor layer including excessive oxygen are formedsuccessively without being exposed to air, whereby an interface statebetween the films each including excessive oxygen can be stabilized, andthe reliability of a TFT can be improved. If the substrate is exposed toair before deposition of the IGZO semiconductor layer, moisture or thelike is attached and the interface state is adversely affected, whichmay cause phenomena such as variation in threshold voltage,deterioration in electric characteristics, and a normally-on TFT.Moisture is a hydrogen compound. When the films are successivelydeposited without being exposed to air, the hydrogen compound can beprevented from existing at the interface. Therefore, By successiveformation of the three layers, variations in threshold voltages can bereduced, deterioration of electric characteristics can be prevented, andshift to normally on of a TFT can be reduced, or preferably eliminated.

In addition, in the second treatment chamber 90 which is a sputteringchamber, both an artificial quartz target and an oxide semiconductortarget containing In, Ga, and Zn are placed, and the films aresuccessively deposited by using a shutter; therefore, the films can bestacked in the same chamber. A shutter is provided between the targetand the substrate; a shutter is opened for a target which is deposited,and a target which is not deposited is shielded by a shutter. Advantagesof stacking the films in the same chamber are that the number ofchambers used can be decreased and attachment of particles or the liketo a substrate can be prevented in transferring the substrate from achamber to another chamber.

Next, the substrate is transferred to the transfer chamber 80 with thetransferring robot 81 by opening the gate valve 86 and transferred to afourth treatment chamber 92 by opening the gate valve 87 without beingexposed to air, and then the gate valve 87 is closed.

Here, the fourth treatment chamber 92 is a sputtering chamber in whichsputtering is performed using an RF magnetron sputtering method. In thefourth treatment chamber 92, a silicon oxide film (S_(i)O_(x) film) isdeposited as a third insulating film 104 to be a channel protectivefilm. Alternatively, for the channel protective film, an aluminum oxidefilm (Al₂O₃ film), a magnesium oxide film (MgO_(x) film), an aluminumnitride film (AlN_(x)) film, an yttrium oxide film (YO_(x)) film, or thelike may be used instead of the silicon oxide film.

A small amount of a halogen element such as fluorine or chlorine may beadded to the channel protective film so as to immobilize movable ionssuch as sodium ions can be fixed. As a method for adding a small amountof a halogen element, sputtering is performed by introducing a gascontaining a halogen element into the chamber. It is to be noted that inthe case where a gas containing a halogen element is introduced, anexhaust means of the chamber is needed to be provided with an abatementsystem for rendering an exhaust gas harmless. The peak of theconcentration of a halogen element to be contained in the gateinsulating film is measured by a secondary ion mass spectrometer (SIMS)and is preferably in the range of from 1×10¹⁵ cm³ to 1×10²⁰ cm⁻³.

When the SiO_(x) film is formed for the channel protective film, asputtering method in which artificial quartz is used as a target and arare gas, typically, argon is used, or a reactive sputtering method inwhich single crystal silicon is used as a target and chemically reactedwith an oxygen gas to obtain a SiO_(x) film can be used. Here,artificial quartz is used as a target, and sputtering is performed underan atmosphere containing only oxygen, or an atmosphere containing oxygenat 90% or more and containing Ar at 10% or less so that a SiO_(x) filmcontains oxygen as much as possible. Thus, the SiO_(x) film containingan excessive amount of oxygen is formed.

By thus successively depositing the SiOx film including excessiveoxygen, the IGZO semiconductor layer including excessive oxygen, and thechannel protective film including excessive oxygen without being exposedto air, the interface state between the films can be stabilized becauseall the three layers contain excessive oxygen, and the reliability of aTFT can be improved. If the substrate is exposed to air beforedeposition of the IGZO semiconductor layer, moisture or the like isattached and the interface state is adversely affected, which may causephenomena such as variation in threshold voltage, degradation inelectric characteristics, and a normally-on TFT. Moisture is a hydrogencompound. When the films are successively deposited without beingexposed to air, the hydrogen compound can be prevented from existing atthe interface. Therefore, by successive formation of the three layers,variations in threshold voltages can be reduced, deterioration ofelectric characteristics can be prevented, and shift to normally on of aTFT can be reduced, or preferably eliminated.

In addition, in the second treatment chamber 90 which is a sputteringchamber, both an artificial quartz target and an oxide semiconductortarget containing In, Ga, and Zn are placed, and the three films aresuccessively deposited by using a shutter; therefore, the films can bestacked in the same chamber. Advantages of stacking the films in thesame chamber are that the number of chambers used can be decreased andattachment of particles, moisture or the like to a substrate can beprevented in transferring the substrate from a chamber to anotherchamber.

In addition, three layers that are the SiOx film including excessiveoxygen, the IGZO semiconductor layer including excessive oxygen, and thechannel protective film including excessive oxygen are successivelyformed, and the layers may be transferred to the first treatment chamber89 and the IGZO semiconductor layer may be subjected to annealing (at300° C. to 400° C.).

After the above steps are conducted repeatedly so that a plurality ofsubstrates in the cassette case are processed for deposition, the vacuumin the cassette chamber is released to air and the substrates and thecassette are taken out. The cross-sectional structure of the substrateat this phase corresponds to the cross-sectional view of the substrateillustrated in FIG. 1B.

Next, the third insulating film 104 is selectively etched to form aninsulator 106 for pattering the semiconductor film 111, and thesemiconductor film 111 is selectively etched to form the IGZOsemiconductor layer 105. Dry etching or wet etching may be used, or twotimes of etchings may be conducted to selectively etch each of them.Note that as a method for etching the semiconductor film 111, an organicacid such as a citric acid or an oxalic acid can be used for an etchant.For example, the semiconductor film 111 with a thickness of 50 nm can beetched in 150 seconds by using the ITO07N (manufactured by KantoChemical Co., inc.). At this phase, in a region with the semiconductorfilm 111 removed, the surface of the gate insulating film is exposed.Here, a second photomask is used. The cross-sectional structure of thesubstrate at this phase corresponds to a cross-sectional view of thesubstrate illustrated in FIG. 1C.

Next, the insulator 106 is etched such that only a part of the insulator106 that overlaps with the gate electrode, that is, the part thatoverlaps with a region to be channel formation region of the IGZOsemiconductor layer 105 is left. A photolithography process is conductedso that selective etching is conducted to the insulator 106. At thistime, a third photomask is used. Here, etching to form a channelprotective film 107 is conducted under the conditions that the etchingrate is sufficiently different from that of the IGZO semiconductorlayer. Note that in the case where the third insulating film 104 isformed using the same material as the gate insulating film, the gateinsulating film is also etched by the etching. Therefore, it ispreferable to use a material different from that of the gate insulatingfilm for the channel protective layer to prevent the gate insulatingfilm from being etched. In this embodiment, the gate insulating film hasa two-layer structure in which the second insulating film 103 as theupper layer is a SiO_(x) film and thus might be removed and the firstinsulating film 102 as the lower layer is a silicon nitride film andfunctions as an etching stopper.

In addition, in the formation of the insulator 106, a resist mask can beformed in a position overlapping with the gate electrode by rear-faceexposure in a self-alignment manner, without using a photomask. Inparticular, the semiconductor film 111 is an oxide semiconductor film,and has a high light-transmitting property and is suitable for rear-faceexposure. Note that in the case of rear-race exposure, all of the firstinsulating film 102, the second insulating film 103, and the thirdinsulating film 104 should include materials having sufficientlight-transmitting properties.

Further, the IGZO semiconductor layer that is exposed is etched by about10 nm using the channel protective film 107 as a mask. In formation ofthe channel protective film (the third insulating film 104) bysputtering, there is a concern that a thin mixed layer might be formedon a surface layer near the interface between the channel protectivefilm (the third insulating film 104) and the IGZO semiconductor layer(the semiconductor film 111), and thus the mixed layer is removed byetching. By removing the mixed layer, good contact resistance can berealized, and thus variation in electric characteristics of TFTs can bereduced. By two times of etchings, the IGZO semiconductor layer ispartially etched on its surface, and a thin region 108 with a thicknesssmaller than the region overlapping with the channel protective film 107is formed. In the IGZO semiconductor layer, the thickness of the thinregion 108 is from 2 nm to 200 nm, preferably from 20 nm to 150 nm. Onetime etching may be conducted to perform both formation of the channelprotective film and partial etching of the surface of the IGZOsemiconductor layer, instead of two times of etchings. Thecross-sectional structure of the substrate at this phase corresponds tothe cross-sectional view of the substrate illustrated in FIG. 1D.

Next, the substrate is set again in the cassette chamber of themultichamber type manufacturing apparatus illustrated in FIG. 2.

Next, after pressure in the cassette chamber is reduced, the substrateis transferred to the transfer chamber 80 and then to the fifthtreatment chamber 93 by opening the gate valve 88.

Here, the fifth treatment chamber 93 is a sputtering chamber using a DCmagnetron sputtering method. In the fifth treatment chamber 93, a metalmultilayer film to be source and drain electrodes is formed. Thesputtering chamber of the fifth treatment chamber 93 is provided withboth a titanium target and an aluminum target and a shutter is used toperform successive formation; thus, films are formed in the samechamber. Here, an aluminum film is stacked over a titanium film andfurther, a titanium film is stacked over the aluminum film.

In addition, the step of etching the IGZO semiconductor layer that isexposed with the channel protective film 107 as a mask by about 10 thickmay be conducted by reverse sputtering in the fifth treatment chamber903, such that the IGZO semiconductor layer in the thin region 108 thatis in contact with the metal multilayer film is not exposed to air. Thereverse sputtering is a method by which voltage is applied to asubstrate side to generate plasma on the substrate side under an argonatmosphere, an oxygen atmosphere or a nitrogen atmosphere, withoutapplying voltage to a target side, so that a surface is etched. In thiscase, the phase at which the channel protective film 107 is set in thecassette chamber of the manufacturing appratus illustrated in FIG. 2. Bynot exposing the IGZO semiconductor layer in the thin region 108 that isin contact with the metal multilayer to air, excellent interface statebetween the IGZO semiconductor layer and the metal multilayer film canbe realized, so that the contact resistance can be reduced.

Alternatively, the reverse sputtering is not conducted in the fifthtreatment chamber, but the surface may be subjected to plasma treatmentor reverse sputtering in the first treatment chamber and transferred tothe fifth treatment chamber without being exposed to air after thetreatment in the first treatment chamber and the metal multilayer filmis formed in the fifth treatment chamber.

Further, heat treatment, specifically, heat treatment at from 300° C. to400° C., preferably, heat treatment at 350° C. or higher can beperformed in the first treatment chamber 89 after deposition of the IGZOfilm including excessive oxygen. By such heat treatment, electriccharacteristics of an inverted staggered thin film transistor can beimproved. Timing of the heat treatment is not limited to a particulartiming as long as the heat treatment is performed after deposition ofthe IGZO film including excessive oxygen. For example, the heattreatment can be performed right after deposition of the IGZO filmincluding excessive oxygen or right after deposition of the metalmulti-layer film. In this case, heat treatment is conducted at 350° C.for one hour after the metal multilayer film is formed.

After the above steps are repeated so that a plurality of substrates inthe cassette case are processed for deposition, the vacuum in thecassette chamber is released to air and the substrates and the cassetteare taken out.

Next, the metal multilayer film is selectively etched to form a sourceelectrode 109 or a drain electrode 110. In this etching, the channelprotective film 107 functions as an etching stopper. Here, a fourthphotomask is used. Note that a conductive film having a three-layer filmin which a titanium film, an aluminum film, and a titanium film aresequentially stacked can be etched by using a hydrogen peroxide solutionor a heated hydrochloric acid as an etchant. The cross-sectionalstructure of the substrate at this phase corresponds to thecross-sectional view of the substrate illustrated in FIG. 1E.

In this embodiment, a multichamber type manufacturing apparatus isdescribed as an example, but an in-line type manufacturing apparatus inwhich sputtering chambers are connected in series may be used andsuccessive formation may be performed without being exposed to air. Inthe case of using the in-line type manufacturing appratus, a substratewhich is vertically placed can be transferred and set vertically in asputtering chamber.

The apparatus illustrated in FIG. 2 has a so-called face-down treatmentchamber in which the deposition target surface of the substrate facesdownward, but may also have a vertical placement treatment chamber inwhich a substrate is placed vertically. The vertical placement treatmentchamber has an advantage that a footprint is smaller than that of aface-down treatment chamber and can be effectively used in the casewhere a large-area substrate which might sags under its weight is used.

Embodiment 2

In Embodiment 2, a case where the thin film transistor obtained inEmbodiment 1 is used as a switching element of a display device isdescribed as an example.

FIG. 3 is a cross-sectional view of a substrate used for a displaydevice. Note that the structure of the thin film transistor has beendescribed in Embodiment 1, and thus detailed description thereof isomitted here.

The thin film transistor includes a gate electrode 401 over a substrate400, and a first insulating film 402 and a second insulating film 403 tocover the gate electrode 401, as illustrated in FIG. 3. In addition, achannel protective film 407 is in contact with a part of an IGZOsemiconductor layer 405, and is arranged over the gate electrode 401 tooverlap therewith. In addition, the IGZO semiconductor layer 405 isprovided between the channel protective film 407 and the gate electrode401.

In addition, as described in Embodiment 1, the metal multilayer filmseach have a three-layer structure in which aluminum films 409 b and 410b are stacked over first titanium films 409 a and 410 a, and secondtitanium films 409 c and 410 c are stacked over the aluminum films 409 band 410 b. Each of the metal multilayer films serves as a sourceelectrode or a drain electrode.

The first titanium films 409 a and 410 a can each have an excellentinterface with the IGZO semiconductor layer so that contact resistanceis low. Further, the aluminum films can realize low-resistant wirings.

The second titanium films 409 c and 410 c can effectively preventhillocks of the aluminum films.

In this case, the thin film transistor obtained in Embodiment 1 iscovered with a protective film or a planarization insulating film inorder to reduce surface unevenness of the thin film transistor andenhance reliability of the thin film transistor. Note that theprotective film is provided to prevent entry of a contaminant impuritysuch as an organic substance, a metal substance or moisture in theatmosphere and is preferably a dense film. The protective film may beformed with a single layer or a multilayer of a silicon oxide film, asilicon nitride film, a silicon oxynitride film and/or a silicon nitrideoxide film formed by a CVD method or the like. In addition, as theprotective film, a silicon oxide film may be formed by a plasma CVDmethod using an organosilane gas and oxygen as a process gas.

As examples of the organosilane, the following compounds can be given:tetraethoxysilane (TEOS) (chemical formula: Si(OC₂H₅)₄),tetramethylsilane (TMS) (chemical formula: Si(CH₃)₄),tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane(OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (chemical formula:SiH(OC₂H₅)₃), trisdimethylaminosilane (chemical formula: SiH(N(CH₃)₂)₃),and the like.

A fourth insulating film 412 is formed as the first layer of theprotective film. The fourth insulating film 412 can effectively preventhillocks of the aluminum film. In this case, a silicon oxide film isformed by a plasma CVD method as the fourth insulating film 412. For aprocess gas for forming the silicon oxide film, TEOS and O₂ are used.For example, the flow of TEOS and O₂ are 15 (sccm) and 750 (sccm),respectively. The substrate temperature at the deposition step is 300°C.

Further, a fifth insulating film 413 is formed as the second layer ofthe protective film. In this case, as the fifth protective film 413, asilicon nitride film is formed by a plasma CVD method. As a process gasfor formation of the silicon nitride film, SiH₄, N₂, NH₃ and H₂ areused. The use of the silicon nitride film as one layer of the protectivefilm can prevent mobile ions of sodium or the like from entering asemiconductor region so that variation in electric characteristics ofTFTs can be suppressed.

Additionally, the IGZO semiconductor layer may be annealed (at 300° C.to 400° C.) after the protective film is formed.

Moreover, a sixth insulating film 414 is formed as the planarizationinsulating film. As the sixth insulating film 414, an organic materialhaving heat resistance, such as acrylic, polyimide, benzocyclobutene,polyamide, or epoxy, can be used. In addition to such organic materials,it is also possible to use a low-dielectric constant material (a low-kmaterial), a siloxane resin, PSG (phosphosilicate glass), BPSG(borophosphosilicate glass), or the like. A siloxane-based resin mayinclude as a substituent at least one of fluorine, an alkyl group, andan aryl group, as well as hydrogen. Note that the sixth insulating film414 may be formed by stacking a plurality of insulating films formedusing from materials.

Note that a siloxane resin is a resin formed from a siloxane material asa starting material and having the bond of Si—O—Si. The siloxane-basedresin may include as a substituent at least one of fluorine, an alkylgroup, and aromatic hydrocarbon, as well as hydrogen.

The sixth interlayer insulating film 414 can be formed by a method suchas CVD, sputtering, SOG, spin coating, dipping, spray coating, dropletdischarging (ink jetting, screen printing, offset printing, or thelike), a doctor knife, a roll coater, a curtain coater, or a knifecoater, depending on the material thereof. When the sixth insulatingfilm 414 is formed using a material liquid, the IGZO semiconductor layermay be annealed (at 300° C. to 400° C.) at the same time as a bakingprocess for the sixth insulating film 414. The baking process of thesixth insulating film 414 serves also as the annealing process of theIGZO semiconductor layer, and thereby a display device can bemanufactured efficiently.

Next, for formation of a contact hole that reaches the second titaniumfilm 410 c, a resist mask is formed by a photolithography process. Atthis time, a fifth photomask is used. Selective etching is conductedusing the resist mask as a mask to form the contact hole that reachesthe second titanium film 410 c. Note that a contact hole that reachesthe gate electrode 401 is formed using the same photomask.

Then, a conductive film that is electrically connected to the secondtitanium film 410 c is formed. The conductive film is selectively etchedby a photolithography process to form a conductive layer 415. At thistime, a sixth photomask is used. The conductive layer 415 is used as apixel electrode or a connection electrode. The pixel electrode iselectrically connected to the second titanium film 410 c. In addition,the connection electrode is electrically connected to the gate electrode401.

The substrate obtained through the above steps can be used for a widevariety of types of display devices.

In a case where a transmissive type liquid crystal display device ismanufactured, the conductive layer 415 serves as a pixel electrode, andcan be formed using a light-transmitting conductive material such asindium oxide containing tungsten oxide, indium zinc oxide containingtungsten oxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

Further, in a case where a transparent conductive film such as an ITOfilm is used as the pixel electrode, the transparent conductive film canhave an excellent interface with the second titanium film 409 c or thesecond titanium film 410 c, so that the contact resistance is low.

Furthermore, the pixel electrode can be formed using a conductivecomposition containing a conductive high-molecular material (alsoreferred to as a “conductive polymer”). The pixel electrode formed fromthe conductive composition has preferably a sheet resistance of 10000Ω/square or less and a transmittance of 70% or more at a wavelength of550 nm. Further, the resistivity of the conductive polymer included inthe conductive composition is preferably 0.1 Ω·cm or lower.

As the conductive polymer, a so-called π-electron conjugated conductivepolymer can be used. As examples thereof, polyaniline or a derivativethereof, polypyrrole or a derivative thereof, polythiophene or aderivative thereof, a copolymer of more than two kinds of them, and thelike can be given.

In addition, in a case where a transmissive type liquid crystal displaydevice is manufactured, the pixel electrode corresponds to a displayregion, and a stacked layer located in the display region shouldtransmit light sufficiently, and thus the first insulating film or thesecond insulating film located in the display region may be partiallyremoved.

Meanwhile, in a case where a reflective type liquid crystal displaydevice is manufactured, a pixel electrode is formed using a conductivematerial having reflectivity, for example, a conductive film formed fromtitanium, tungsten, nickel, gold, platinum, silver, copper, tantalum,molybdenum, aluminum, magnesium, calcium, lithium, or an alloy thereof

Moreover, in a case where an EL display device is manufactured, amaterial of a pixel electrode should be selected in consideration of itswork function. In this embodiment, the thin film transistor is ann-channel thin film transistor, and thus the conductive layer 415preferably serves as a cathode, and a material having a low workfunction (typically, a material having a work function of 3.5 eV orless) is preferable. However, with use of an EL layer formed over theconductive layer 415, typically, an organic compound layer having anexcellent hole-injecting or hole-transporting property, or an organiccompound layer having an excellent electron-injecting orelectron-transporting property, the conductive layer 415 can be formedusing any of a variety of materials without particular limitations onits work function.

In addition, in a case where an electrophoretic display device ismanufactured, a pixel electrode can be formed with any conductive filmthat has low electric resistance, without no particular limitations.

This embodiment can be freely combined with Embodiment 1.

Embodiment 3

In a case where a thin film transistor is used as a switching element ofa display device, it is important to reduce leakage current in an offstate. In Embodiment 3, an example of a structure of a thin filmtransistor whose leakage current amount in an off state is reduced isdescribed.

In Embodiment 1, the single gate structure is exemplified, but inEmbodiment 3, a multigate thin film transistor that can reduce moreleakage current amount in its off state is exemplified.

The multigate thin film transistor includes a plurality of channelformation regions. FIG. 4A illustrates an example of a double-gate typethin film transistor.

Embodiment 3 is different only in layout from Embodiment 1, and thus themanufacturing method of the double-gate type thin film transistor is notdescribed here in detail.

FIG. 4A illustrates an example of a double-gate type thin filmtransistor using two IGZO semiconductor layers formed over a substrate600, and the thin film transistor includes a first channel formationregion 605 that overlaps with and is in contact with a first channelprotective film 608 and a second channel formation region 606 thatoverlaps with and is in contact with a second channel protective film607.

The first channel formation region 605 overlaps with a gate electrode601 with a gate insulating film 602 therebetween. In addition, thesecond channel formation region 606 overlaps with the gate electrode 601with the gate insulating film 602 therebetween.

Further, the thickness of regions of the IGZO semiconductor layers,which are in contact with source or drain electrodes 609, 610 is madesmaller than that of the first channel formation region 605 and thesecond channel formation region 606.

A connection electrode 611 that is in contact with both of the firstchannel protective film 608 and the second channel protective film 607is formed over the IGZO semiconductor layers. The connection electrode611 is formed in the same process as the source or drain electrodes 609,610. The connection electrode 611 is a floating electrode. Note that endportions of the two IGZO semiconductor layers are located between thetwo gate electrode 601, and the connection electrode 611 is in contactwith part of the gate insulating film 602.

In addition, the thickness of regions of the IGZO semiconductor layersthat are in contact with the connection electrode 611 is smaller thanthe thickness of the first channel formation region 605 and the secondchannel formation region 606.

The structure illustrated in FIG. 4A can realize reduction of leakagecurrent in an off state. By reduction of leakage current in an offstate, power consumption of a display device can be reduced.

FIG. 4B illustrates a cross-section of a double-gate type thin filmtransistor which is partially different from that in FIG. 4A. FIG. 4Billustrates an example of the double-gate type thin film transistorusing one IGZO semiconductor layer formed over the substrate 600. Thethin film transistor illustrated in FIG. 4B is different from that inFIG. 4A only in that the end portions of the IGZO semiconductor layer donot exist between the two gate electrodes 601. The other portions ofFIG. 4B are the same as those of FIG. 4A, and thus are denoted by thesame numerals as those used in FIG. 4A. The thin film transistor of FIG.4A and the thin film transistor of FIG. 4B have substantially the sameelectric characteristics.

This embodiment can be freely combined with Embodiment 1 or Embodiment2.

Embodiment 4

In an example of Embodiment 4 to be described below, at least a part ofa driver circuit and a thin film transistor to be disposed in a pixelportion are formed over one substrate.

The thin film transistor to be disposed in the pixel portion is formedaccording to any one of Embodiments 1 to 3. Further, the thin filmtransistor described in any one of Embodiments 1 to 3 is an n-channelTFT, and thus a part of a driver circuit that can include an n-channelTFT among driver circuits is formed over the same substrate as the thinfilm transistor of the pixel portion.

FIG. 5A illustrates an example of a block diagram of an active matrixliquid crystal display device. The display device illustrated in FIG. 5Aincludes, over a substrate 5300, a pixel portion 5301 including aplurality of pixels that are provided with a display element; a scanline driver circuit 5302 that selects a pixel; and a signal line drivercircuit 5303 that controls a video signal input to the selected pixel.

The pixel portion 5301 is connected to the signal line driver circuit5303 by a plurality of signal lines S1 to Sm (not illustrated) thatextend in a column direction from the signal line driver circuit 5303,and to the scan line driver circuit 5302 by a plurality of scan lines G1to Gn (not illustrated) that extend in a row direction from the scanline driver circuit 5302. The pixel portion 5301 includes a plurality ofpixels (not illustrated) arranged in matrix so as to correspond to thesignal lines S1 to Sm and the scan lines G1 to Gn. Each pixel isconnected to a signal line Sj (one of the signal lines S1 to Sm) and ascan line Gj (one of the scan lines G1 to Gn).

In addition, the thin film transistor described in any one ofEmbodiments 1 to 3 is an n-channel TFT, and a signal line driver circuitincluding the n-channel TFT is described with reference to FIG. 6.

The signal line driver circuit illustrated in FIG. 6 includes a driverIC 5601, switch groups 5602_1 to 5602_M, a first wiring 5611, a secondwiring 5612, a third wiring 5613, and wirings 5621_1 to 5621_M. Each ofthe switch groups 5602_1 to 5602_M includes a first thin film transistor5603 a, a second thin film transistor 5603 b, and a third thin filmtransistor 5603 c.

The driver IC 5601 is connected to the first wiring 5611, the secondwiring 5612, the third wiring 5613, and the wirings 5621_1 to 5621_M.Each of the switch groups 5602_1 to 5602_M is connected to the firstwiring 5611, the second wiring 5612, and the third wiring 5613, and theswitch groups 5602_1 to 5602_M are connected to the wirings 5621_1 to5621_M, respectively. Each of the wirings 5621_1 to 5621_M is connectedto three signal lines via the first thin film transistor 5603 a, thesecond thin film transistor 5603 b, and the third thin film transistor5603 c. For example, the wiring 5621_J of the J-th column (one of thewirings 5621_1 to 5621_M) is connected to a signal line Sj−1, a signalline Sj, and a signal line Sj+1 via the first thin film transistor 5603a, the second thin film transistor 5603 b, and the third thin filmtransistor 5603 c included in the switch group 5602_J.

A signal is input to each of the first wiring 5611, the second wiring5612, and the third wiring 5613.

Note that the driver IC 5601 is preferably formed over a singlecrystalline substrate. The switch groups 5602_1 to 5602_M are preferablyformed over the same substrate as thin film transistors in the pixelportion illustrated in any one of Embodiments 1 to 3. Therefore, thedriver IC 5601 and the switch groups 5602_1 to 5602_M are preferablyconnected through an FPC or the like.

Next, operation of the signal line driver circuit illustrated in FIG. 6is described with reference to a timing chart in FIG. 7. The timingchart in FIG. 7 illustrates a case where the scan line Gi of the i-throw is selected. A selection period of the scan line Gi of the i-th rowis divided into a first sub-selection period T1, a second sub-selectionperiod T2, and a third sub-selection period T3. In addition, the signalline driver circuit in FIG. 6 operates as in FIG. 7 even when a scanline of another row is selected.

Note that the timing chart in FIG. 7 shows a case where the wiring5621_J in the J-th column is connected to the signal line Sj−1, thesignal line Sj, and the signal line Sj+1 via the first thin filmtransistor 5603 a, the second thin film transistor 5603 b, and the thirdthin film transistor 5603 c.

The timing chart in FIG. 7 shows timing at which the scan line Gi of thei-th row is selected, timing 5703 a of on/off of the first thin filmtransistor 5603 a, timing 5703 b of on/off of the second thin filmtransistor 5603 b, timing 5703 c of on/off of the third thin filmtransistor 5603 c, and a signal 5721_J input to the wiring 5621_J of theJ-th column.

In the first sub-selection period T1, the second sub-selection periodT2, and the third sub-selection period T3, different video signals areinput to the wirings 5621_1 to 5621_M. For example, a video signal inputto the wiring 5621_J in the first sub-selection period T1 is input tothe signal line Sj−1, a video signal input to the wiring 5621_J in thesecond sub-selection period T2 is input to the signal line Sj, and avideo signal input to the wiring 5621_J in the third sub-selectionperiod T3 is input to the signal line Sj+1. In addition, in the firstsub-selection period T1, the second sub-selection period T2, and thethird sub-selection period T3, the video signals input to the wiring5621_J are denoted by Data_j−1, Data_j, and Data_j+1.

As illustrated in FIG. 7, in the first sub-selection period T1, thefirst thin film transistor 5603 a is turned on, and the second thin filmtransistor 5603 b and the third thin film transistor 5603 c are turnedoff. At this time, Data_j−1 input to the wiring 5621_J is input to thesignal line Sj−1 via the first thin film transistor 5603 a. In thesecond sub-selection period T2, the second thin film transistor 5603 bis turned on, and the first thin film transistor 5603 a and the thirdthin film transistor 5603 c are turned off. At this time, Data_j inputto the wiring 5621_J is input to the signal line Sj via the second thinfilm transistor 5603 b. In the third sub-selection period T3, the thirdthin film transistor 5603 c is turned on, and the first thin filmtransistor 5603 a and the second thin film transistor 5603 b are turnedoff. At this time, Data_j+1 input to the wiring 5621_J is input to thesignal line Sj+1 via the third thin film transistor 5603 c.

As described above, in the signal line driver circuit in FIG. 6, bydividing one gate selection period into three, video signals can beinput to three signal lines from one wiring 5621 in one gate selectionperiod. Therefore, in the signal line driver circuit in FIG. 6, thenumber of connections of the substrate provided with the driver IC 5601and the substrate provided with the pixel portion can be approximately ⅓of the number of signal lines. The number of connections is reduced toapproximately ⅓ of the number of the signal lines, so that reliability,yield, etc., of the signal line driver circuit in FIG. 6 can beimproved.

Note that there are no particular limitations on the arrangement, thenumber, a driving method, and the like of the thin film transistors, aslong as one gate selection period is divided into a plurality ofsub-selection periods and video signals are input to a plurality ofsignal lines from one wiring in the respective the sub-selection periodsas illustrated in FIG. 6.

For example, when video signals are input to three or more signal linesfrom one wiring in each of three or more sub-selection periods, it isonly necessary to add a thin film transistor and a wiring forcontrolling the thin film transistor. Note that when one gate selectionperiod is divided into four or more sub-selection periods, onesub-selection period becomes shorter. Therefore, one gate selectionperiod is preferably divided into two or three sub-selection periods.

As another example, one gate selection period may be divided into aprecharge period Tp, the first sub-selection period T1, the secondsub-selection period T2, and the third sub-selection period T3 asillustrated in a timing chart in FIG. 8. The timing chart in FIG. 8illustrates timing at which the scan line Gi of the i-th row isselected, timing 5803 a of on/off of the first thin film transistor 5603a, timing 5803 b of on/off of the second thin film transistor 5603 b,timing 5803 c of on/off of the third thin film transistor 5603 c, and asignal 5821_J input to the wiring 5621_J of the J-th column. Asillustrated in FIG. 8, the first thin film transistor 5603 a, the secondthin film transistor 5603 b, and the third thin film transistor 5603 care tuned on in the precharge period Tp. At this time, precharge voltageVp input to the wiring 5621_J is input to each of the signal line Sj−1,the signal line Sj, and the signal line Sj+1 via the first thin filmtransistor 5603 a, the second thin film transistor 5603 b, and the thirdthin film transistor 5603 c. In the first sub-selection period T1, thefirst thin film transistor 5603 a is turned on, and the second thin filmtransistor 5603 b and the third thin film transistor 5603 c are turnedoff. At this time, Data_j−1 input to the wiring 5621_J is input to thesignal line Sj−1 via the first thin film transistor 5603 a. In thesecond sub-selection period T2, the second thin film transistor 5603 bis turned on, and the first thin film transistor 5603 a and the thirdthin film transistor 5603 c are turned off. At this time, Data_j inputto the wiring 5621_J is input to the signal line Sj via the second thinfilm transistor 5603 b. In the third sub-selection period T3, the thirdthin film transistor 5603 c is turned on, and the first thin filmtransistor 5603 a and the second thin film transistor 5603 b are turnedoff. At this time, Data j+1 input to the wiring 5621_J is input to thesignal line Sj+1 via the third thin film transistor 5603 c.

As described above, in the signal line driver circuit in FIG. 6 to whichthe timing chart in FIG. 8 is applied, the video signal can be writtento the pixel at high speed because the signal line can be precharged byproviding a precharge selection period before a sub-selection period.Note that portions in FIG. 8 which are similar to those of FIG. 7 aredenoted by common reference numerals and detailed description of theportions which are the same and portions which have similar functions isomitted.

Further, a structure of a scan line driver circuit is described. Thescan line driver circuit includes a shift register and a buffer.Additionally, the scan line driver circuit may include a level shifterin some cases. In the scan line driver circuit, when the clock signal(CLK) and the start pulse signal (SP) are input to the shift register, aselection signal is produced. The generated selection signal is bufferedand amplified by the buffer, and the resulting signal is supplied to acorresponding scan line. Gate electrodes of transistors in pixels of oneline are connected to the scan line. Further, since the transistors inthe pixels of one line have to be turned on at the same time, a bufferwhich can feed a large current can be used.

One mode of a shift register which is used for a part of a scan linedriver circuit is described with reference to FIG. 9 and FIG. 10.

FIG. 9 illustrates a circuit configuration of the shift register. Theshift register illustrated in FIG. 9 includes a plurality of flip-flops(flip-flops 5701-1 to 5701-n). The shift register is operated with inputof a first clock signal, a second clock signal, a start pulse signal,and a reset signal.

Connection relations of the shift register in FIG. 9 are described. Inthe i-th stage flip-flop 5701 _(—) i (one of the flip-flops 5701-1 to5701 _(—) n) in the shift register of FIG. 9, a first wiring 5501illustrated in FIG. 10 is connected to a seventh wiring 5717 _(—) i−1; asecond wiring 5502 illustrated in FIG. 10 is connected to a seventhwiring 5717 _(—) i+1; a third wiring 5503 illustrated in FIG. 10 isconnected to a seventh wiring 5717 _(—) i; and a sixth wiring 5506illustrated in FIG. 10 is connected to a fifth wiring 5715.

Further, a fourth wiring 5504 illustrated in FIG. 10 is connected to asecond wiring 5712 in flip-flops of odd-numbered stages, and isconnected to a third wiring 5713 in flip-flops of even-numbered stages.A fifth wiring 5505 illustrated in FIG. 10 is connected to a fourthwiring 5714.

Note that the first wiring 5501 of the first stage flip-flop 5701_1illustrated in FIG. 10 is connected to a first wiring 5711. Moreover,the second wiring 5502 of the n-th stage flip-flop 5701 _(—) nillustrated in FIG. 10 is connected to a sixth wiring 5716.

Note that the first wiring 5711, the second wiring 5712, the thirdwiring 5713, and the sixth wiring 5716 may be referred to as a firstsignal line, a second signal line, a third signal line, and a fourthsignal line, respectively. The fourth wiring 5714 and the fifth wiring5715 may be referred to as a first power source line and a second powersource line, respectively.

Next, FIG. 10 illustrates details of the flip-flop illustrated in FIG.9. A flip-flop illustrated in FIG. 10 includes a first thin filmtransistor 5571, a second thin film transistor 5572, a third thin filmtransistor 5573, a fourth thin film transistor 5574, a fifth thin filmtransistor 5575, a sixth thin film transistor 5576, a seventh thin filmtransistor 5577, and an eighth thin film transistor 5578. Each of thefirst thin film transistor 5571, the second thin film transistor 5572,the third thin film transistor 5573, the fourth thin film transistor5574, the fifth thin film transistor 5575, the sixth thin filmtransistor 5576, the seventh thin film transistor 5577, and the eighththin film transistor 5578 is an n-channel transistor and is turned onwhen the gate-source voltage (V_(gs)) exceeds the threshold voltage(V_(th)).

Next, the connection structure of the flip-flop illustrated in FIG. 9 isdescribed below.

A first electrode (one of a source electrode and a drain electrode) ofthe first thin film transistor 5571 is connected to the fourth wiring5504. A second electrode (the other of the source electrode and thedrain electrode) of the first thin film transistor 5571 is connected tothe third wiring 5503.

A first electrode of the second thin film transistor 5572 is connectedto the sixth wiring 5506. A second electrode of the second thin filmtransistor 5572 is connected to the third wiring 5503.

A first electrode of the third thin film transistor 5573 is connected tothe fifth wiring 5505. A second electrode of the third thin filmtransistor 5573 is connected to a gate electrode of the second thin filmtransistor 5572. A gate electrode of the third thin film transistor 5573is connected to the fifth wiring 5505.

A first electrode of the fourth thin film transistor 5574 is connectedto the sixth wiring 5506. A second electrode of the fourth thin filmtransistor 5574 is connected to the gate electrode of the second thinfilm transistor 5572. A gate electrode of the fourth thin filmtransistor 5574 is connected to a gate electrode of the first thin filmtransistor 5571.

A first electrode of the fifth thin film transistor 5575 is connected tothe fifth wiring 5505. A second electrode of the fifth thin filmtransistor 5575 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the fifth thin film transistor5575 is connected to the first wiring 5501.

A first electrode of the sixth thin film transistor 5576 is connected tothe sixth wiring 5506. A second electrode of the sixth thin filmtransistor 5576 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the sixth thin film transistor5576 is connected to the gate electrode of the second thin filmtransistor 5572.

A first electrode of the seventh thin film transistor 5577 is connectedto the sixth wiring 5506. A second electrode of the seventh thin filmtransistor 5577 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the seventh thin filmtransistor 5577 is connected to the second wiring 5502. A firstelectrode of the eighth thin film transistor 5578 is connected to thesixth wiring 5506. A second electrode of the eighth thin film transistor5578 is connected to the gate electrode of the second thin filmtransistor 5572. A gate electrode of the eighth thin film transistor5578 is connected to the first wiring 5501.

Note that the points at which the gate electrode of the first thin filmtransistor 5571, the gate electrode of the fourth thin film transistor5574, the second electrode of the fifth thin film transistor 5575, thesecond electrode of the sixth thin film transistor 5576, and the secondelectrode of the seventh thin film transistor 5577 are connected areeach referred to as a node 5543. The points at which the gate electrodeof the second thin film transistor 5572, the second electrode of thethird thin film transistor 5573, the second electrode of the fourth thinfilm transistor 5574, the gate electrode of the sixth thin filmtransistor 5576, and the second electrode of the eighth thin filmtransistor 5578 are connected are each referred to as a node 5544.

Note that the first wiring 5501, the second wiring 5502, the thirdwiring 5503, and the fourth wiring 5504 may be referred to as a firstsignal line, a second signal line, a third signal line, and a fourthsignal line, respectively. The fifth wiring 5505 and the sixth wiring5506 may be referred to as a first power source line and a second powersource line, respectively.

In addition, the signal line driver circuit and the scan line drivercircuit can be formed using only the n-channel TFTs described in any oneof Embodiments 1 to 3. The n-channel TFT described in any one ofEmbodiments 1 to 3 has a high mobility in the transistorcharacteristics, and thus a driving frequency of a driver circuit can beincreased. For example, a scan line driver circuit using the n-channelTFT described in any one of Embodiments 1 to 3 can operate at highspeed, and thus a frame frequency can be increased and insertion ofblack images can be realized.

In addition, when the channel width of the transistor in the scan linedriver circuit is increased or a plurality of scan line driver circuitsare provided, for example, higher frame frequency can be realized. Whena plurality of scan line driver circuits are provided, a scan linedriver circuit for driving even-numbered scan lines is provided on oneside and a scan line driver circuit for driving odd-numbered scan linesis provided on the opposite side; thus, increase in frame frequency canbe realized.

Further, when an active matrix light-emitting display device ismanufactured, a plurality of transistors are arranged in at least onepixel, and thus a plurality of scan line driver circuits are preferablyarranged. FIG. 5B is a block diagram illustrating an example of anactive matrix light-emitting display device.

The display device illustrated in FIG. 5B includes, over a substrate5400, a pixel portion 5401 having a plurality of pixels provided with adisplay element, a first scan line driver circuit 5402 and a second scanline driver circuit 5404 that select a pixel, and a signal line drivercircuit 5403 that controls input of a video signal to the selectedpixel.

When the video signal input to a pixel of the display device illustratedin FIG. 5B is a digital signal, a pixel emits light or does not emitlight by switching of ON/Off of a transistor. Thus, grayscale can bedisplayed using an area ratio grayscale method or a time ratio grayscalemethod. An area ratio grayscale method refers to a driving method bywhich one pixel is divided into a plurality of subpixels and therespective subpixels are driven independently based on video signals sothat grayscale is displayed. Further, a time ratio grayscale methodrefers to a driving method by which a period during which a pixel is ina light-emitting state is controlled so that grayscale is displayed.

Since the response speed of light-emitting elements is higher than thatof liquid crystal elements or the like, the light-emitting elements aremore suitable for a time ratio grayscale method than liquid-crystaldisplay elements. Specifically, in the case of displaying with a timegray scale method, one frame period is divided into a plurality ofsubframe periods. Then, in accordance with video signals, thelight-emitting element in the pixel is set in a light-emitting state ora non-light-emitting state in each subframe period. By dividing oneframe into a plurality of subframes, the total length of time, in whichpixels actually emit light in one frame period, can be controlled withvideo signals so that gray scales are displayed.

In the example of the light-emitting device illustrated in FIG. 5B, in acase where two TFTs of a switching TFT and a current control TFT arearranged in one pixel, the first scan line driver circuit 5402 generatesa signal which is input to a first scan line serving as a gate wiring ofthe switching TFT, and the second scan line driver circuit 5404generates a signal which is input to a second scan line serving as agate wiring of the current control TFT; however, one scan line drivercircuit may generate both the signal which is input to the first scanline and the signal which is input to the second scan line. In addition,for example, there is a possibility that a plurality of the first scanlines used for controlling the operation of the switching element areprovided in each pixel, depending on the number of transistors includedin the switching element. In that case, one scan line driver circuit maygenerate all signals that are input to the plurality of first scanlines, or a plurality of scan line driver circuits may generate signalsthat are input to the plurality of first scan lines.

In addition, also in the light-emitting device, a part of the drivercircuit(s) that can include n-channel TFTs among driver circuits can beformed over the same substrate as the thin film transistors of the pixelportions. Alternatively, the signal line driver circuit and the scanline driver circuit can be formed using only the n-channel TFTsdescribed in Embodiment 1 or 2.

Moreover, the above-described driver circuit can be used for electronicpaper that drives electronic ink using an element electrically connectedto a switching element, without being limited to applications to aliquid crystal display device or a light-emitting device. The electronicpaper is also referred to as an electrophoretic display device(electrophoretic display) and has advantages in that it has the samelevel of readability as plain paper, it has lower power consumption thanother display devices, and it can be made thin and lightweight.

Electrophoretic displays can have various modes. Electrophoreticdisplays contain a plurality of microcapsules dispersed in a solvent ora solute, each microcapsule containing first particles which arepositive-charged and second particles which are negative-charged. Byapplying an electric field to the microcapsules, the particles in themicrocapsules are moved in opposite directions and only the color of theparticles concentrated on one side is exhibited. It is to be noted thatthe first particles and the second particles each contain pigment and donot move without an electric field. Moreover, the colors of the firstparticles and the second particles are different from each other (thecolors include colorless or achroma).

In this way, an electrophoretic display is a display that utilizes aso-called dielectrophoretic effect by which a substance that has a highdielectric constant move to a high-electric field region. Anelectrophoretic display does not need to use a polarizer and a countersubstrate, which are required in a liquid crystal display device, andboth the thickness and weight of the electrophoretic display device canbe a half of those of a liquid crystal display device.

A solution in which the aforementioned microcapsules are dispersed in asolvent is referred to as electronic ink. This electronic ink can beprinted on a surface of glass, plastic, cloth, paper, or the like.Furthermore, by use of a color filter or particles that have a pigment,color display is possible, as well.

In addition, if a plurality of the aforementioned microcapsules arearranged as appropriate over an active matrix substrate so as to beinterposed between two electrodes, an active matrix display device canbe completed, and display can be performed by application of an electricfield to the microcapsules. For example, the substrate obtained inEmbodiment 2 can be used.

It is to be noted that the first particles and the second particles inthe microcapsules may each be formed of a single material selected froma conductive material, an insulator material, a semiconductor material,a magnetic material, a liquid crystal material, a ferroelectricmaterial, an electroluminescent material, an electrochromic material, ora magnetophoretic material or formed of a composite material of any ofthese.

This embodiment can be freely combined with any one of Embodiment Modes1 to 3.

Embodiment 5

A thin film transistor of one embodiment of the present invention ismanufactured, and a display device having a display function can bemanufactured using the thin film transistor for a pixel portion andfurther for a driver circuit. Further, part or whole of a driver circuitcan be formed over the same substrate as a pixel portion, using a thinfilm transistor of one embodiment of the present invention, whereby asystem-on-panel can be obtained.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement) or a light-emitting element (also referred to as alight-emitting display element) can be used. Light-emitting elementsinclude, in its category, an element whose luminance is controlled bycurrent or voltage, and specifically include an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Further, a display medium whose contrast is changed by an electriceffect, such as an electronic ink, can be used.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC including a controller orthe like is mounted on the panel. One embodiment of the presentinvention relates to one mode of an element substrate before the displayelement is completed in a manufacturing process of the display device,and the element substrate is provided with means for supplying currentto the display element in each of a plurality of pixels. Specifically,the element substrate may be in a state provided with only a pixelelectrode of the display element, a state after a conductive film to bea pixel electrode is formed and before the conductive film is etched toform the pixel electrode, or any of other states.

Note that the term “display device” in this specification means an imagedisplay device, a display device, or a light source (including alighting device). Further, the display device includes any of thefollowing modules in its category: a module to which a connector such asa flexible printed circuit (FPC), tape automated bonding (TAB) tape, ora tape carrier package (TCP) is attached; a module having TAB tape or aTCP which is provided with a printed wiring board at the end thereof;and a module having an integrated circuit (IC) which is directly mountedon a display element by a chip on glass (COG) method.

In this embodiment, a liquid crystal display device will be described asan example of a semiconductor device of one embodiment of the presentinvention.

FIGS. 11A and 11B illustrate an active-matrix liquid crystal displaydevice to which one embodiment of the present invention is applied. FIG.11A is a plan view of the liquid crystal display device. FIG. 11B is across-sectional view taken along line V-X of FIG. 11A. A thin filmtransistor 201 used in the semiconductor device can be manufactured in amanner similar to the thin film transistor described in Embodiment 1 andis a highly reliable thin film transistor formed at reduced cost bysuccessive deposition using a sputtering method. Any of the thin filmtransistors described in Embodiments 2 and 3 can also be used as thethin film transistor 201 of this embodiment.

The liquid crystal display device of this embodiment illustrated in FIG.11A includes a source wiring layer 202, a thin film transistor 201 witha multi-gate structure, a gate wiring layer 203, and a capacitor wiringlayer 204.

Further, in FIG. 11B, in the liquid crystal display device of thisembodiment, a substrate 200 provided with the thin film transistor 201with a multi-gate structure, an insulating layer 211, an insulatinglayer 212, an insulating layer 213, an electrode layer 255 used for adisplay element, an insulating layer 261 serving as an alignment film,and a polarizing plate 268 and a substrate 266 provided with aninsulating layer 263 serving as an alignment film, an electrode layer265 used for a display element, a coloring layer 264 serving as a colorfilter, and a polarizing plate 267 face to each other with a liquidcrystal layer 262 interposed therebetween; thus, a liquid crystaldisplay element 260 is formed.

Alternatively, as the liquid crystal layer 262, liquid crystal showing ablue phase for which an alignment film is unnecessary may be used. Ablue phase is one of liquid crystal phases, which is generated justbefore a cholesteric phase changes into an isotropic phase whiletemperature of cholesteric liquid crystal is increased. Since the bluephase is generated within an only narrow range of temperature, liquidcrystal composition in which a chiral agent at 5 wt % or more is mixedis used for the liquid crystal layer 262 in order to improve thetemperature range. The liquid crystal composition which includes liquidcrystal showing a blue phase and a chiral agent have suchcharacteristics that the response time is as short as 10 μs to 100 μs,the alignment process is unnecessary because the liquid crystalcomposition has optical isotropy, and viewing angle dependency is small.

While FIGS. 11A and 11B illustrate an example of the liquid crystaldisplay device in which the polarizing plate 267 is provided in aposition outer than the substrate 266 (on the viewer side) and thecoloring layer 264 and the electrode layer 265 used for a displayelement are provided in a position inner than the substrate 266 in thatorder, the polarizing plate 267 may be provided in an inner positionthan the substrate 266. The stacked structure of the polarizing plateand the coloring layer is not limited to that shown in FIG. 11B and maybe set as appropriate depending on materials of the polarizing plate andthe coloring layer or conditions of a manufacturing process. Further, alight-blocking film serving as a black matrix may be provided.

The electrode layers 255 and 265 each serving as a pixel electrode layercan be formed using a light-transmitting conductive material such asindium oxide including tungsten oxide, indium zinc oxide includingtungsten oxide, indium oxide including titanium oxide, indium tin oxideincluding titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, indium tin oxide to which silicon oxide isadded, or the like.

Through this process, a highly reliable liquid crystal display device asa semiconductor device can be manufactured.

This embodiment can be combined with any structure described in theother embodiments, as appropriate.

Embodiment 6

In Embodiment 6, an example of electronic paper will be described as asemiconductor device according to one embodiment of the presentinvention.

FIG. 12 illustrates active matrix electronic paper as an example of asemiconductor device to which one embodiment of the present invention isapplied. A thin film transistor 581 used for the semiconductor devicecan be manufactured in a manner similar to the thin film transistordescribed in Embodiment 1 and is a highly reliable thin film transistorformed at reduced cost by successive deposition using a sputteringmethod. Any of the thin film transistors described in Embodiments 2 and3 can also be used as the thin film transistor 201 of this embodiment.

The electronic paper in FIG. 12 is an example of a display device usinga twisting ball display system. The twisting ball display system refersto a method in which spherical particles each colored in black and whiteare arranged between a first electrode layer and a second electrodelayer which are electrode layers used for a display element, and apotential difference is generated between the first electrode layer andthe second electrode layer to control orientation of the sphericalparticles, so that display is performed.

The thin film transistor 581 is an inverted staggered thin filmtransistor with a multi-gate structure, and a source electrode and adrain electrode thereof is in contact with a first electrode layer 587at an opening formed in an insulating layer 585, whereby the thin filmtransistor 581 is electrically connected to the first electrode layer587. Between the first electrode layer 587 and a second electrode layer588, spherical particles 589 each having a black region 590 a, a whiteregion 590 b, and a cavity 594 around the regions which is filled withliquid are provided. A space around the spherical particles 589 isfilled with a filler 595 such as a resin (see FIG. 12).

Further, instead of the twisting ball, an electrophoretic element canalso be used. A microcapsule having a diameter of about 10 μm to 200 μmin which transparent liquid, positively charged white microparticles,and negatively charged black microparticles are encapsulated, is used.In the microcapsule which is provided between the first electrode layerand the second electrode layer, when an electric field is applied by thefirst electrode layer and the second electrode layer, the whitemicroparticles and black microparticles move to opposite sides, so thatwhite or black can be displayed. A display element using this principleis an electrophoretic display element and is called electronic paper ingeneral. The electrophoretic display element has higher reflectance thana liquid crystal display element, and thus, an auxiliary light isunnecessary, power consumption is low, and a display portion can berecognized in a dim place. In addition, even when power is not suppliedto the display portion, an image which has been displayed once can bemaintained. Accordingly, a displayed image can be stored even if asemiconductor device having a display function (which may be referred tosimply as a display device or a semiconductor device provided with adisplay device) is distanced from an electric wave source.

Through this process, highly reliable electronic paper as asemiconductor device can be manufactured.

This embodiment can be combined with the structure described in otherembodiments, as appropriate.

Embodiment 7

In this embodiment, an example of a light-emitting display device willbe described as a semiconductor device according one embodiment of thepresent invention. As a display element included in a display device, alight-emitting element utilizing electroluminescence is described here.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus, the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that description ismade here using an organic EL element as a light-emitting element.

FIGS. 13A and 13B illustrate an active-matrix light-emitting displaydevice as an example of a semiconductor device to which one embodimentof the present invention is applied. FIG. 13A is a plan view of thelight-emitting display device, and FIG. 13B is a cross-sectional viewtaken along line Y-Z of FIG. 13A. FIG. 14 shows an equivalent circuit ofthe light-emitting display device illustrated in FIGS. 13A and 13B.

Thin film transistors 301 and 302 used for a semiconductor device can bemanufactured in a manner similar to the thin film transistor describedin Embodiment 1 and are highly reliable thin film transistors formed atreduced cost by successive deposition using a sputtering method. Thethin film transistors described in Embodiments 2 and 3 can also be usedas the thin film transistors 301 and 302 of this embodiment.

The light-emitting display device of this embodiment illustrated in FIG.13A and FIG. 14 includes a thin film transistor 301 with a multi-gatestructure, a thin film transistor 302, a light-emitting element 303, acapacitor element 304, a source wiring layer 305, a gate wiring layer306, and a power source line 307. The thin film transistors 301 and 302are n-channel thin film transistors.

In FIG. 13B, the light-emitting display device of this embodimentincludes the thin film transistor 302; an insulating layer 311; aninsulating layer 312; an insulating layer 313; a partition wall 321; anda first electrode layer 320, an electroluminescent layer 322, and asecond electrode layer 323 which are used for the light-emitting element303.

The insulating layer 313 is preferably formed using an organic resinsuch as acrylic, polyimide, or polyamide or using siloxane. Since thethin film transistor 302 in the pixel is n-type in this embodiment, as acathode, the first electrode layer 320 which is a pixel electrode layeris preferably used. Specifically, for the cathode, a material with a lowwork function such as Ca, Al, CaF, MgAg, or AlLi can be used.

The partition wall 321 is formed using an organic resin film, aninorganic insulating film, or organic polysiloxane. Particularlypreferably, the partition wall 321 is formed using a photosensitivematerial and an opening be formed over the first electrode layer 320 sothat a sidewall of the opening is formed as an inclined surface withcontinuous curvature.

The electroluminescent layer 322 may be formed using a single layer or aplurality of layers stacked.

The second electrode layer 323 used as an anode is formed to cover theelectroluminescent layer 322. The second electrode layer 323 can beformed using a light-transmitting conductive film using any of thelight-transmitting conductive materials listed in Embodiment 5 for thepixel electrode layer. The second electrode layer 323 may also be formedusing a titanium nitride film or a titanium film instead of theabove-described light-transmitting conductive film. The light-emittingelement 303 is formed by overlapping of the first electrode layer 320,the electroluminescent layer 322, and the second electrode layer 323.After that, a protective film may be formed over the second electrodelayer 323 and the partition wall 321 in order to prevent entry ofoxygen, hydrogen, moisture, carbon dioxide, or the like into thelight-emitting element 303. As the protective film, a silicon nitridefilm, a silicon nitride oxide film, a DLC film, or the like can beformed.

Further, in a practical case, preferably, a display device completed inthe state illustrated in FIG. 13B is packaged (sealed) with a protectivefilm (such as a laminate film or an ultraviolet curable resin film) or acover material with high air-tightness and little degasification so thatthe display device is not exposed to the outside air.

Next, structures of the light-emitting element will be described withreference to FIGS. 15A to 15C. A cross-sectional structure of a pixelwill be described by taking an n-channel driving TFT as an example.Driving TFTs 7001, 7011, and 7021 used for semiconductor devicesillustrated in FIGS. 15A to 15C can be manufactured in a manner similarto the thin film transistor described in Embodiment 1 and are highlyreliable thin film transistors formed at reduced cost by successivedeposition using a sputtering method. Alternatively, any of the thinfilm transistors described in Embodiments 2 and 3 can be employed as theTFTs 7001, 7011, and 7021.

In order to extract light emitted from the light-emitting element, atleast one of the anode and the cathode is required to transmit light. Athin film transistor and a light-emitting element are formed over asubstrate. A light-emitting element can have a top emission structure,in which light emission is extracted through the surface opposite to thesubstrate; a bottom emission structure, in which light emission isextracted through the surface on the substrate side; or a dual emissionstructure, in which light emission is extracted through the surfaceopposite to the substrate and the surface on the substrate side. Thepixel structure according to one embodiment of the present invention canbe applied to a light-emitting element having any of these emissionstructures.

A light-emitting element having a top emission structure will bedescribed with reference to FIG. 15A.

FIG. 15A is a cross-sectional view of a pixel in the case where thedriving TFT 7001 is of an n-type and light is emitted from alight-emitting element 7002 to an anode 7005 side. In FIG. 15A, acathode 7003 of the light-emitting element 7002 is electricallyconnected to the driving TFT 7001, and a light-emitting layer 7004 andthe anode 7005 are stacked in this order over the cathode 7003. Thecathode 7003 can be formed using a variety of conductive materials aslong as they have a low work function and reflect light. Thelight-emitting layer 7004 may be formed using a single layer or aplurality of layers stacked. When the light-emitting layer 7004 isformed using a plurality of layers, the light-emitting layer 7004 isformed by stacking an electron-injecting layer, an electron-transportinglayer, a light-emitting layer, a hole-transporting layer, and ahole-injecting layer in this order over the cathode 7003. It is notnecessary to form all of these layers. The anode 7005 is formed using alight-transmitting conductive film.

The light-emitting element 7002 corresponds to a region where thecathode 7003 and the anode 7005 sandwich the light-emitting layer 7004.In the case of the pixel illustrated in FIG. 15A, light is emitted fromthe light-emitting element 7002 to the anode 7005 side as indicated byan arrow.

Next, a light-emitting element having a bottom emission structure willbe described with reference to FIG. 15B. FIG. 15B is a cross-sectionalview of a pixel in the case where the driving TFT 7011 is of an n-typeand light is emitted from a light-emitting element 7012 to a cathode7013 side. In FIG. 15B, the cathode 7013 of the light-emitting element7012 is formed over a light-transmitting conductive film 7017 that iselectrically connected to the driving TFT 7011, and a light-emittinglayer 7014 and an anode 7015 are stacked in this order over the cathode7013. A light-blocking film 7016 for reflecting or blocking light may beformed to cover the anode 7015 when the anode 7015 has alight-transmitting property. For the cathode 7013, various materials canbe used as in the case of FIG. 15A as long as they are conductivematerials having a low work function. The cathode 7013 is formed to havea thickness that can transmit light (preferably, approximately 5 nm to30 nm). For example, an aluminum film with a thickness of 20 nm can beused as the cathode 7013. Similar to the case of FIG. 15A, thelight-emitting layer 7014 may be formed using either a single layer or aplurality of layers stacked. The anode 7015 is not required to transmitlight, but can be formed using a light-transmitting conductive materialas in the case of FIG. 15A. As the light-blocking film 7016, a metal orthe like that reflects light can be used for example; however, it is notlimited to a metal film. For example, a resin or the like to which blackpigments are added can also be used.

The light-emitting element 7012 corresponds to a region where thecathode 7013 and the anode 7015 sandwich the light-emitting layer 7014.In the case of the pixel illustrated in FIG. 15B, light is emitted fromthe light-emitting element 7012 to the cathode 7013 side as indicated byan arrow.

Next, a light-emitting element having a dual emission structure will bedescribed with reference to FIG. 15C. In FIG. 15C, a cathode 7023 of alight-emitting element 7022 is formed over a light-transmittingconductive film 7027 which is electrically connected to the driving TFT7021, and a light-emitting layer 7024 and an anode 7025 are stacked inthis order over the cathode 7023. As in the case of FIG. 15A, thecathode 7023 can be formed using a variety of conductive materials aslong as they have a low work function. The cathode 7023 is formed tohave a thickness that can transmit light. For example, a film of Alhaving a thickness of 20 nm can be used as the cathode 7023. As in FIG.15A, the light-emitting layer 7024 may be formed using either a singlelayer or a plurality of layers stacked. The anode 7025 can be formedusing a light-transmitting conductive material as in the case of FIG.15A.

The light-emitting element 7022 corresponds to a region where thecathode 7023, the light-emitting layer 7024, and the anode 7025 overlapwith one another. In the case of the pixel illustrated in FIG. 15C,light is emitted from the light-emitting element 7022 to both the anode7025 side and the cathode 7023 side as indicated by arrows.

Note that, although an organic EL element is described here as alight-emitting element, an inorganic EL element can also be provided asa light-emitting element.

In this embodiment, the example is described in which a thin filmtransistor (a driving TFT) which controls the driving of alight-emitting element is electrically connected to the light-emittingelement; however, a structure may be employed in which a TFT for currentcontrol is connected between the driving TFT and the light-emittingelement.

A semiconductor device described in this embodiment is not limited tothe structures illustrated in FIGS. 15A to 15C and can be modified invarious ways based on the spirit of techniques according to the presentinvention.

Through the above-described process, a highly reliable light-emittingdisplay device as a semiconductor device can be manufactured.

This embodiment can be combined with any structure described in theother embodiments, as appropriate.

Embodiment 8

Next, a structure of a display panel, which is one embodiment of thesemiconductor device of the present invention, will be described below.In this embodiment, a liquid crystal display panel (also referred to asa liquid crystal panel), which is one embodiment of a liquid crystaldisplay device having a liquid crystal element as a display element, anda light-emitting display panel (also referred to as a light-emittingpanel), which is one embodiment of a semiconductor device having alight-emitting element as a display element, will be described.

Next, the appearance and a cross section of a light-emitting displaypanel, which is one embodiment of the semiconductor device of thepresent invention, will be described with reference to FIGS. 16A and16B. FIG. 16A is a top view of a panel in which a highly reliable thinfilm transistor formed at reduced cost by successive deposition using asputtering method are sealed between a first substrate and a secondsubstrate with a sealant. FIG. 16B is a cross-sectional view taken alongline H-I of FIG. 16A.

A sealant 4505 is provided so as to surround a pixel portion 4502,signal line driver circuits 4503 a and 4503 b, and scan line drivercircuits 4504 a and 4504 b which are provided over a first substrate4501. In addition, a second substrate 4506 is provided over the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b. Accordingly, the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b are sealed together with afiller 4507, by the first substrate 4501, the sealant 4505, and thesecond substrate 4506.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b formed over thefirst substrate 4501 each include a plurality of thin film transistors,and a thin film transistor 4510 included in the pixel portion 4502 and athin film transistor 4509 included in the signal line driver circuit4503 a are illustrated as an example in FIG. 16B.

Each of the thin film transistors 4509 and 4510 corresponds to a thinfilm transistor formed at reduced cost by successive deposition using asputtering method, and any of the thin film transistors described inEmbodiments 2 and 3 can be applied thereto. In this embodiment, the thinfilm transistors 4509 and 4510 are n-channel thin film transistors.

Moreover, reference numeral 4511 denotes a light-emitting element. Afirst electrode layer 4517 which is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a sourceelectrode layer or a drain electrode layer of the thin film transistor4510. Note that the structure of the light-emitting element 4511 is notlimited to that described in this embodiment. The structure of thelight-emitting element 4511 can be changed as appropriate depending onthe direction in which light is extracted from the light-emittingelement 4511, or the like.

In addition, a variety of signals and potentials are supplied to thesignal line driver circuits 4503 a and 4503 b, the scan line drivercircuits 4504 a and 4504 b, or the pixel portion 4502 from FPCs 4518 aand 4518 b.

The connection terminal 4515 is formed with the same conductive film asthe second electrode layer 4512, the wiring 4516 is formed with the sameconductive film as the first electrode layer 4517 included in thelight-emitting element 4511.

The connection terminal 4515 is electrically connected to a terminalincluded in the FPC 4518 a through an anisotropic conductive film 4519.

The second substrate 4506 located in the direction in which light isextracted from the light-emitting element 4511 needs to have alight-transmitting property. In that case, a light-transmitting materialsuch as a glass plate, a plastic plate, a polyester film, or an acrylicfilm is used.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used, in addition to an inert gas such as nitrogen orargon. For example, PVC (polyvinyl chloride), acrylic, polyimide, anepoxy resin, a silicone resin, PVB (polyvinyl butyral), or EVA (ethylenevinyl acetate) can be used. In this embodiment, nitrogen is used for thefiller 4507.

In addition, if needed, an optical film, such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

As the signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b, driver circuits formed using a singlecrystal semiconductor film or polycrystalline semiconductor film over asubstrate separately prepared may be mounted. Alternatively, only thesignal line driver circuits or part thereof, or the scan line drivercircuits or part thereof may be separately formed and mounted. Thisembodiment is not limited to the structure illustrated in FIGS. 16A and16B.

Next, the appearance and a cross section of a liquid crystal displaypanel, which is one embodiment of the semiconductor device of thepresent invention, will be described with reference to FIGS. 17A1, 17A2and 17B. FIGS. 17A1 and 17A2 are top views of a panel in which highlyreliable thin film transistors 4010 and 4011 each having an IGZOsemiconductor layer and an IGZO semiconductor layer having an n-typeconductivity, and a liquid crystal element 4013 are sealed between afirst substrate 4001 and a second substrate 4006 with a sealant 4005.FIG. 17B is a cross-sectional view taken along line M-N of FIGS. 17A1and 17A2.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 which are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scan line driver circuit 4004. Therefore, the pixelportion 4002 and the scan line driver circuit 4004 are sealed togetherwith liquid crystal layer 4008, by the first substrate 4001, the sealant4005, and the second substrate 4006. A signal line driver circuit 4003that is formed using a single crystal semiconductor film or apolycrystalline semiconductor film over a substrate separately preparedis mounted in a region that is different from the region surrounded bythe sealant 4005 over the first substrate 4001.

Note that the connection method of a driver circuit which is separatelyformed is not particularly limited, and a COG method, a wire bondingmethod, a TAB method, or the like can be used. FIG. 17A1 illustrates anexample of mounting the signal line driver circuit 4003 by a COG method,and FIG. 17A2 illustrates an example of mounting the signal line drivercircuit 4003 by a TAB method.

The pixel portion 4002 and the scan line driver circuit 4004 providedover the first substrate 4001 include a plurality of thin filmtransistors. FIG. 17B exemplifies the thin film transistor 4010 includedin the pixel portion 4002 and the thin film transistor 4011 included inthe scan line driver circuit 4004.

Each of the thin film transistors 4010 and 4011 corresponds to a thinfilm transistor formed at reduced cost by successive deposition using asputtering method, and any of the thin film transistors described inEmbodiments 2 and 3 can be employed as the thin film transistors 4010and 4011. In this embodiment, the thin film transistors 4010 and 4011are n-channel thin film transistors.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is formed on thesecond substrate 4006. A portion where the pixel electrode layer 4030,the counter electrode layer 4031, and the liquid crystal layer 4008overlap with one another corresponds to the liquid crystal element 4013.Note that the pixel electrode layer 4030 and the counter electrode layer4031 are provided with an insulating layer 4032 and an insulating layer4033 respectively which each function as an alignment film, and sandwichthe liquid crystal layer 4008 with the insulating layers 4032 and 4033interposed between the pixel electrode layer 4030 and the counterelectrode layer 4031.

Note that the first substrate 4001 and the second substrate 4006 can beformed by using glass, metal (typically, stainless steel), ceramic, orplastic. As plastic, a fiberglass-reinforced plastics (FRP) plate, apolyvinyl fluoride (PVF) film, a polyester film, or an acrylic resinfilm can be used. In addition, a sheet with a structure in which analuminum foil is sandwiched between PVF films or polyester films can beused.

Reference numeral 4035 denotes a columnar spacer obtained by selectivelyetching an insulating film and is provided to control the distancebetween the pixel electrode layer 4030 and the counter electrode layer4031 (a cell gap). Further, a spherical spacer may also be used.

Further, a variety of signals and potentials are supplied to the signalline driver circuit 4003 which is formed separately, the scan linedriver circuit 4004, or the pixel portion 4002 from an FPC 4018.

In this embodiment, a connection terminal 4015 is formed using the sameconductive film as that of the pixel electrode layer 4030 included inthe liquid crystal element 4013, and a wiring 4016 is formed using thesame conductive film as that of gate electrode layers of the thin filmtransistors 4010 and 4011.

The connection terminal 4015 is electrically connected to a terminalincluded in the FPC 4018 through an anisotropic conductive film 4019.

Note that FIGS. 17A1, 17A2 and 17B illustrate an example in which thesignal line driver circuit 4003 is formed separately and mounted on thefirst substrate 4001; however, this embodiment is not limited to thisstructure. The scan line driver circuit may be separately formed andthen mounted, or only part of the signal line driver circuit or part ofthe scan line driver circuit may be separately formed and then mounted.

FIG. 18 illustrates an example in which a liquid crystal display moduleis formed as a semiconductor device by using a TFT substrate 2600manufactured according to one embodiment of the present invention.

FIG. 18 illustrates an example of a liquid crystal display module, inwhich the TFT substrate 2600 and a counter substrate 2601 are fixed toeach other with a sealant 2602, and a pixel portion 2603 including a TFTor the like, a display element 2604 including a liquid crystal layer,and a coloring layer 2605 are provided between the substrates to form adisplay region. The coloring layer 2605 is necessary to perform colordisplay. In the case of the RGB system, respective coloring layerscorresponding to colors of red, green, and blue are provided forrespective pixels. Polarizing plates 2606 and 2607 and a diffusion plate2613 are provided outside the TFT substrate 2600 and the countersubstrate 2601. A light source includes a cold cathode tube 2610 and areflective plate 2611, and a circuit substrate 2612 is connected to awiring circuit portion 2608 of the TFT substrate 2600 through a flexiblewiring board 2609 and includes an external circuit such as a controlcircuit or a power source circuit. The polarizing plate and the liquidcrystal layer may be stacked with a retardation plate interposedtherebetween.

For the liquid crystal display module, a TN (twisted nematic) mode, anIPS (in-plane-switching) mode, an FFS (fringe field switching) mode, anMVA (multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASM (axially symmetric aligned micro-cell) mode, anOCB (optical compensated birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode,or the like can be used.

Through the above-described process, a highly reliable display panel asa semiconductor device can be manufactured.

This embodiment can be combined with any structure described in theother embodiments, as appropriate.

Embodiment 9

A semiconductor device according to one embodiment of the presentinvention can be applied to a variety of electronic devices (includingan amusement machine). Examples of electronic devices are a televisionset (also referred to as a television or a television receiver), amonitor of a computer or the like, a camera such as a digital camera ora digital video camera, a digital photo frame, a mobile phone handset(also referred to as a mobile phone or a mobile phone device), aportable game console, a portable information terminal, an audioreproducing device, a large-sized game machine such as a pachinkomachine, and the like.

FIG. 19A illustrates an example of a portable information terminaldevice 9200. The portable information terminal device 9200 incorporatesa computer and thus can process various types of data. An example of theportable information terminal device 9200 is a personal digitalassistant.

The portable information terminal device 9200 has two housings, ahousing 9201 and a housing 9203. The housing 9201 and the housing 9203are joined with a joining portion 9207 such that the portableinformation terminal device 9200 can be foldable. A display portion 9202is incorporated in the housing 9201, and the housing 9203 includes akeyboard 9205. Needless to say, the structure of the portableinformation terminal device 9200 is not limited to the above structure,and the structure may should include at least a thin film transistorhaving a channel protective layer according to one embodiment of thepresent invention, and additional accessory may be provided asappropriate. According to one embodiment of the present invention, ahighly reliable portable information terminal device formed at low costcan be provided.

FIG. 19B illustrates an example of a digital video camera 9500. Thedigital video camera 9500 includes a display portion incorporated in ahousing 9501 and various operation portions. Needless to say, thestructure of the digital video camera 9500 is not limited to the abovestructure, and the structure should include at least a thin filmtransistor having a channel protective layer according to one embodimentof the present invention, and additional accessory may be provided asappropriate. According to one embodiment of the present invention, ahighly reliable digital video camera formed at low cost can be provided.

FIG. 19C illustrates an example of a cellular phone 9100. The cellularphone 9100 has two housings, a housing 9102 and a housing 9101. Thehousing 9102 and the housing 9101 are joined with a joining portion 9103such that the cellular phone is foldable. A display portion 9104 isincorporated in the housing 9102, and the housing 9101 includesoperation keys 9106. Needless to say, the structure of the cellularphone 9100 is not limited to the above structure, and the structureshould include at least a thin film transistor having a channelprotective layer according to one embodiment of the present invention,and additional accessory may be provided as appropriate. According toone embodiment of the present invention, a highly reliable cellularphone formed at low cost can be provided.

FIG. 19D illustrates an example of a portable computer 9400. Thecomputer 9400 has two housings, a housing 9401 and a housing 9404. Thehousing 9401 and the housing 404 are joined such that the computer canbe open and closed. A display portion 9402 is incorporated in thehousing 9401, and the housing 9404 includes a key board 9403. Needlessto say, the structure of the computer 9400 is not limited to the abovestructure, and the structure should include at least a thin filmtransistor having a channel protective layer according to one embodimentof the present invention, and additional accessory may be provided asappropriate. According to one embodiment of the present invention, ahighly reliable computer formed at low cost can be provided.

FIG. 20 illustrates an example of a cellular phone 1000 which isdifferent from that illustrated in FIG. 19C. The cellular phone 1000includes a display portion 1002 incorporated in a housing 1001, anoperation button 1003, an external connection port 1004, a speaker 1005,a microphone 1006 and the like.

In the cellular phone 1000 illustrated in FIG. 20, data can be inputwhen a person touches the display portion 1002 with his/her finger orthe like. In addition, operations such as phone call or mailing can beconducted when a person touches the display portion 1002 with his/herfinger or the like.

There are mainly three screen modes of the display portion 1002: thefirst mode is a display mode mainly for displaying an image; the secondmode is an input mode mainly for inputting data such as text; and thethird mode is a display-and-input mode in which two modes of the displaymode and the input mode are combined.

For example, in the case of calling or mailing, a text input mode mainlyfor inputting text is selected for the display portion 1002 so that textdisplayed on a screen can be input. In that case, it is preferable todisplay a keyboard or number buttons on almost all area of the screen ofthe display portion 1002.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone 1000, display in the screen of the display portion 1002 canbe automatically switched by determining the direction of the mobilephone 1000 (whether the mobile phone 1000 is placed horizontally orvertically for a landscape mode or a portrait mode).

The screen modes are switched by touching the display portion 1002 oroperating the operation button 1003 of the housing 1001. Alternatively,the screen modes may be switched depending on the kind of the imagedisplayed on the display portion 1002. For example, when a signal of animage displayed on the display portion is a signal of moving image data,the screen mode is switched to the display mode. When the signal is asignal of text data, the screen mode is switched to the input mode.

Further, in the input mode, when input by touching the display portion1002 is not performed for a certain period while a signal detected bythe optical sensor in the display portion 1002 is detected, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

FIG. 21A illustrates an example of a television set 9600. In thetelevision set 9600, a display portion 9603 is incorporated in a housing9601. The display portion 9603 can display an image. Further, thehousing 9601 is supported by a stand 9605 here. There are no particularlimitations on the structure of the television set 9600. The structuremay have at least a thin film transistor including a channel protectivelayer according to one embodiment of the present invention, and mayinclude addition equipment as appropriate. A highly reliable televisionset formed at low cost can be realized according to one embodiment ofthe present invention.

The television set 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller 9610. Channels and volumecan be controlled by an operation key 9609 of the remote controller 9610so that an image displayed on the display portion 9603 can becontrolled. Further, the remote controller 9610 may be provided with adisplay portion 9607 for displaying data output from the remotecontroller 9610.

Note that the television set 9600 is provided with a receiver, a modem,and/or the like. With the receiver, a general television broadcast canbe received. Further, when the television set 9600 is connected to acommunication network by wired or wireless connection via the modem,one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver or between receivers) data communication canbe performed.

FIG. 21B illustrates an example of a digital photo frame 9700. Forexample, in the digital photo frame 9700, a display portion 9703 isincorporated in a housing 9701. The display portion 9703 can displayvarious images. For example, the display portion 9703 can display dataof an image shot by a digital camera or the like to function as a normalphoto frame. There are no particular limitations on the structure of thedigital photo frame 9700. The structure may have at least a thin filmtransistor including a channel protective layer according to oneembodiment of the present invention, and may include addition equipmentas appropriate. A highly reliable digital photo frame formed at low costcan be realized according to one embodiment of the present invention.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection portion (a USB terminal, a terminal thatcan be connected to various cables such as a USB cable, or the like), arecording medium insertion portion, and the like. Although they may beprovided on the surface on which the display portion is provided, it ispreferable to provide them on the side surface or the back surface forthe design of the digital photo frame 9700. For example, a memorystoring data of an image shot by a digital camera is inserted in therecording medium insertion portion of the digital photo frame, wherebythe image data can be transferred into the digital photo frame 9700 anddisplayed on the display portion 9703.

The digital photo frame 9700 may transmit and receive data wirelessly.The structure may be employed in which desired image data is transferredinto the digital photo frame 9700 wirelessly to be displayed.

A thin film transistor having a channel protective layer according toone embodiment of the present invention can be applied to electronicpaper. The electronic paper can be used for electronic devices ofvarious fields, which display information. For example, the electronicpaper can be applied to an electronic book device (including e-bookreaders and electronic books), posters, advertisements on vehicles suchas trains, displays on various cards such as credit cards, or the like.Examples of the electronic devices are illustrated in FIG. 22 and FIG.23.

FIG. 22 is an example of an electronic book device 2700. For example,the electronic book device 2700 includes two housings, a housing 2701and a housing 2703. The housings 2701 and 2703 are connected by a hingeportion 2711 and can be opened or closed with the hinge portion 2711.With such a structure, the electronic book device 2700 can be handledlike a book made of paper.

A display portion 2705 is incorporated in the housing 2701 and a displayportion 2707 is incorporated in the housing 2703. The display portions2705 and 2707 may display one image or different images separately. Witha structure in which different images are displayed, for example, textis displayed on the display portion on the right side (the displayportion 2705 in FIG. 22) while an image is displayed on the displayportion on the left side (the display portion 2707 in FIG. 22).

FIG. 22 illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power source 2721, an operation key 2723, a speaker2725, and the like. With the operation key 2723, pages which aredisplayed are changed. Note that a keyboard, a pointing device, or thelike may be provided in the same plane as that of the display portion ofthe housing. In addition, a rear surface or a side surface of thehousing may be provided with an external connection terminal (anearphone terminal, a USB terminal, a terminal which can be connected toan AC adapter or various cables such as a USB cable, or the like), arecording medium insert portion, or the like. Further, the electronicbook device 2700 may have a function as an electronic dictionary.

Note that the electronic book device 2700 may have a configurationcapable of wirelessly transmitting and receiving data. With wirelesscommunication, desired book data or the like can be purchased anddownloaded from servers for electronic books. There are no particularlimitations on the structure of the electronic book device 2700. Thestructure may have at least a thin film transistor including a channelprotective layer according to one embodiment of the present invention,and may include addition equipment as appropriate. An electronic bookdevice formed at low cost can be realized according to one embodiment ofthe present invention.

FIG. 23 illustrates a car card advertising device 3602 on a train or thelike. In a case where an advertising medium is paper, a man replacesadvertising, but in a case where it is electronic paper, much manpoweris not needed and replacement of advertising can be conducted in shorttime. Further, images can be displayed stably without breaking adisplayed image. In addition, information of the advertising may betransmitted and received wirelessly. There are no particular limitationson the structure of such advertizing device. The structure may have atleast a thin film transistor including a channel protective layeraccording to one embodiment of the present invention, and may includeaddition equipment as appropriate. A highly reliable advertising deviceformed at low cost can be realized according to one embodiment of thepresent invention.

This embodiment can be freely combined with any of Embodiments 1 to 8.This application is based on Japanese Patent Application serial no.2008-197127 filed with Japan Patent Office on Jul. 31, 2008, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A display device comprising: a transistor comprising: agate electrode over a substrate; a first insulating layer comprisingsilicon nitride over the gate electrode; a second insulating layercomprising silicon oxide over the first insulating layer; an oxidesemiconductor layer comprising a channel formation region overlappingthe gate electrode with the first insulating layer and the secondinsulating layer therebetween; a third insulating layer comprisingsilicon oxide over the channel formation region; a source electrode anda drain electrode over the third insulating layer; a fourth insulatinglayer comprising silicon oxide over the source electrode and the drainelectrode; and a fifth insulating layer comprising an organic materialover the fourth insulating layer; a light-emitting element comprising: afirst electrode over the fifth insulating layer; an electroluminescentlayer over the first electrode; and a second electrode over theelectroluminescent layer; and a protective film comprising siliconnitride over the light-emitting element, wherein the first electrode iselectrically connected to one of the source electrode and the drainelectrode of the transistor.
 3. The display device according to claim 2,wherein the first electrode is in direct contact with the one of thesource electrode and the drain electrode of the transistor.
 4. Thedisplay device according to claim 2, wherein the transistor has asingle-gate structure.
 5. The display device according to claim 2,further comprising a sixth insulating layer comprising silicon nitridebetween the fourth insulating layer and the sixth insulating layer.
 6. Adisplay module including the display device according to claim
 2. 7. Atelevision including the display device according to claim
 2. 8. Adisplay device comprising a pixel comprising: a first transistor; asecond transistor; a capacitor; and a light-emitting element, whereinone of a source and a drain of the first transistor is electricallyconnected to a source wiring, wherein the other of the source and thedrain of the first transistor is electrically connected to one electrodeof the capacitor, wherein a gate of the second transistor iselectrically connected to the one electrode of the capacitor, whereinone of a source and a drain of the second transistor is electricallyconnected to the light-emitting element, wherein the other of the sourceand the drain of the second transistor is electrically connected to apower source line, wherein each of the first transistor and the secondtransistor comprise: a gate electrode over a substrate; a firstinsulating layer comprising silicon nitride over the gate electrode; asecond insulating layer comprising silicon oxide over the firstinsulating layer; an oxide semiconductor layer comprising a channelformation region overlapping the gate electrode with the firstinsulating layer and the second insulating layer therebetween; a thirdinsulating layer comprising silicon oxide over the channel formationregion; a source electrode and a drain electrode over the thirdinsulating layer; a fourth insulating layer comprising silicon oxideover the source electrode and the drain electrode; and a fifthinsulating layer comprising an organic material over the fourthinsulating layer, wherein the light-emitting element comprising: a firstelectrode over the fifth insulating layer; an electroluminescent layerover the first electrode; and a second electrode over theelectroluminescent layer, and wherein the first electrode iselectrically connected to one of the source electrode and the drainelectrode of the second transistor.
 9. The display device according toclaim 8, wherein the one electrode of the capacitor comprises the samematerial and is provided on the same surface as the source electrode andthe drain electrode, and wherein the other electrode of the capacitorcomprises the same material and is provided on the same surface as thegate electrode.
 10. The display device according to claim 8, wherein thefirst electrode is in direct contact with the one of the sourceelectrode and the drain electrode of the second transistor.
 11. Thedisplay device according to claim 8, wherein the first transistor andthe second transistor each have a single-gate structure.
 12. The displaydevice according to claim 8, further comprising a sixth insulating layercomprising silicon nitride between the fourth insulating layer and thesixth insulating layer.
 13. A display module including the displaydevice according to claim
 8. 14. A television including the displaydevice according to claim
 8. 15. A display device comprising a pixelcomprising: a first transistor; a second transistor; a capacitor; and alight-emitting element, wherein one of a source and a drain of the firsttransistor is electrically connected to a source wiring, wherein theother of the source and the drain of the first transistor iselectrically connected to one electrode of the capacitor, wherein a gateof the second transistor is electrically connected to the one electrodeof the capacitor, wherein one of a source and a drain of the secondtransistor is electrically connected to the light-emitting element,wherein the other of the source and the drain of the second transistoris electrically connected to a power source line, wherein each of thefirst transistor and the second transistor comprise: a gate electrodecomprising copper over a substrate; a first insulating layer comprisingsilicon nitride over the gate electrode; a second insulating layercomprising silicon oxide over the first insulating layer; an oxidesemiconductor layer comprising a channel formation region comprisingindium, gallium, and zinc and overlapping the gate electrode with thefirst insulating layer and the second insulating layer therebetween; athird insulating layer comprising silicon oxide over the channelformation region; a source electrode and a drain electrode comprisingtitanium over the third insulating layer; a fourth insulating layercomprising silicon oxide over the source electrode and the drainelectrode; and a fifth insulating layer comprising an organic materialover the fourth insulating layer, wherein the light-emitting elementcomprising: a first electrode comprising indium tin oxide over the fifthinsulating layer; an electroluminescent layer over the first electrode;and a second electrode comprising aluminum over the electroluminescentlayer, wherein a protective film comprising silicon nitride is providedover the light-emitting element, and wherein the first electrode iselectrically connected to one of the source electrode and the drainelectrode of the second transistor.
 16. The display device according toclaim 15, wherein the one electrode of the capacitor comprises the samematerial and is provided on the same surface as the source electrode andthe drain electrode, and wherein the other electrode of the capacitorcomprises the same material and is provided on the same surface as thegate electrode.
 17. The display device according to claim 15, whereinthe first electrode is in direct contact with the one of the sourceelectrode and the drain electrode of the second transistor.
 18. Thedisplay device according to claim 15, wherein the first transistor andthe second transistor each have a single-gate structure.
 19. The displaydevice according to claim 15, further comprising a sixth insulatinglayer comprising silicon nitride between the fourth insulating layer andthe sixth insulating layer.
 20. A display module including the displaydevice according to claim
 15. 21. A television including the displaydevice according to claim 15.